PRESCRIPTION FOR SUCCESS\Surgeon-author operated on own portfolio, bypassing diversification.

Byline: Geraldine Fabrikant The New York Times

The young surgeon had it all: degrees from the best schools, a thriving practice, rugged good looks, an active social life. Yet, it was not enough; he wanted much more.

That, after all, was why he was sitting in this classroom at Harvard University's Kennedy Center School for Public Policy, half listening to the professor hold forth on Colorado River water rights and half watching the falling snow envelop the Cambridge campus outside. For his dream was to go into politics, starting with a seat in the state senate and ending, perhaps . . .

Suddenly, his reverie was shattered by his beeper. He assumed it was a call from his office, summoning him to emergency surgery.

The doctor was dead wrong . . .

No, this is not the beginning of one of Dr. Robin Cook's 15 best-selling novels. But it was the start of the medical-mystery writer's investment dilemma.

The phone call in March 1977 was from his literary agent telling Cook that the paperback rights to "Coma," a thriller about organ transplants, had just been sold for $800,000.

That was an astronomical sum at the time, especially since the hardcover version of the book hadn't even come out yet. When it was published a couple of months later, it did well, and the paperback that followed leaped onto the best-seller lists.

That success was followed by a film version, which grossed more than $50 million and made a small fortune for Cook.

His predicament was figuring out what to do with all that sudden wealth. With limited investment experience, Cook presumably needed the advice of professionals. But he distrusted their motives.

"When you get even a small success, you are besieged with people who want to be your business manager, and I refused everything," the writer, now a silver-haired 55, recalled recently at his six-story town house in Boston's Beacon Hill district. "I felt that there wasn't anybody who would be more careful with any money I might earn than myself."

He was also convinced that having advisers "was the worst circumstance to get yourself into because you got wedded to this group of people who were helping you and they were really helping themselves. You realize that people are out for their own best interests. If it serves your ends it is OK, but it is not their main goal. So you have to take everything that is brought to you with a grain of salt."

So Cook took the plunge unaided - and promptly violated one of the cardinal rules recited by investment specialists: diversify.

Rather than build a broad-based portfolio with allocations to stocks, to bonds, to money-market funds and so forth, he put most of his eggs in a single basket: real estate. What he didn't put into property, he put into cash.

His first move was the purchase of the home in Beacon Hill's elegant Louisburg Square for $165,000 in 1977 - then a rather princely sum. That same year, he bought a little condo in New Hampshire.

In the next several years, he lavished $1 million on improving the Beacon Hill town house and also paid a total of $1.3 million for three condos in Naples, Fla., two of which he later resold.

His biggest real estate purchases, though, were to come in the 1990s: a condo in Naples for $1.8 million in 1990; a condo in Trump Tower on Fifth Avenue in Manhattan for $2.1 million in 1993 and a retreat on Martha's Vineyard, the Massachusetts resort island, for $1.4 million in 1995.

"I think Robin has a place everywhere but Yugoslavia," quipped his friend and fellow best-selling author, Robert Ludlum.

While the bull market gathered steam on Wall Street, Cook shied away from stocks, making his first foray only in 1985. He has branched out into stocks and bonds in the past decade - but without much enthusiasm.

"Designing and living in houses for me is extremely enjoyable," he said, "whereas stocks and bonds aren't so much fun."

So does he regret missing out on much of the stock boom of the 1980s? Not at all. "I was getting 20 percent on my cash," he said. "Better for me to put my time into what makes money for me - writing - than in going after the absolute maximum return."

All in all, roughly half of the estimated $15 million he has earned from his writing and from TV and movie versions of his books has gone into property.

Cook also swam against the tide of conventional financial wisdom in the way he financed his real-estate acquisitions: Unlike the vast majority of home buyers in this country who relish the tax deductibility of mortgage interest, he paid cash almost every time. He explained that he was willing to speculate in real estate, but didn't want to take on debt to do so.

"The most risk you can have is being overextended," he said. "I pay cash for everything. Taking out a mortgage is being a gambler. It entails anxiety because you have extended yourself out.

"Let's say - a worst-case scenario - the Florida market fell apart. I would not owe anything, even if the house burned down."

Spilling across the dining-room table in Cook's Boston town house are floor plans for the house on Martha's Vineyard, where he spent last Sunday working with architects.

Casually dressed in jeans and a T-shirt, Cook spoke easily about his plans. He said he had already spent more than $1 million on the Beacon Hill town house. "One on the square just sold for over $4 million," he said with relish.

And he is now gutting the Trump Tower apartment. "I have torn every possible thing out because it was built so poorly," Cook said. "If they had tried, they could not have made a worse floor plan." It doesn't bother him that neither he nor anyone else has spent a single night at the apartment.

The decision to invest in stocks wasn't so much an investment epiphany as an exercise in pragmatism. It was 1985, and after six years of marriage, Cook was in the midst of a divorce. His lawyer in Naples, GeorSge Vega, took a look at his portfolio, mostly certificates of deposit and cash during the bull market, and gasped.

"It was very strange," Vega said. "I told Robin he had better things to do with his money and that he ought to check into something a little broader: stocks, bonds or anything."

Cook took the advice - but not necessarily because he agreed with his lawyer's belief in the importance of a diversified portfolio.

Cook put his liquid assets with Suntrust, a bank based in Naples. By now, he said, he has invested approximately as much in the account - which he described as a "conservative, broad mixture in stocks, bonds and other investments" - as he has put into real estate. And, Cook said, he has done "very well."

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Photo (Color) Dr. Robin Cook has authored 15 best-selling novels. The New York Times Chart One Man's Real Estate Portfolio The luxury homes of novelist Robin Cook. The New York Times