Galling stones: fill 'er up.

Methyl tert-butyl ether (MTBE), a chemical included in gasoline to prevent engine knocking, has been used to dissolve gallstones in six people at the Mayo Clinic in Rochester, Minn. The liquid, which dissolves the cholesterol deposits that compose eight out of every 10 gallstones, was infused through a tube into the gallbladder for an average of six to 12 hours.

Gallstones cause acute pain under the ribs on the right side of the body and in rare cases can be fatal. About 500,000 U.S. patients have their gallstone-forming gallbladders removed each year. While surgery is "safe and very effective," notes Gerald R. May of the Mayo Clinic, it carries with it expense and a two- to four-week convalescence. "We're hoping that the [MTBE] procedure will be cheaper, and that people will be out of the hospital in two days," he says. But first, he notes, further evaluation is necessary to see if MTBE gets into and harms other parts of the body and if stones recur. Toxicity determinations in animal studies gave researchers enough confidence in MTBE's safety to begin testing in humans--where, so far, no side effects have been seen.