Patients with cholesterol gallstones referred to elective cholecystectomy were randomly assigned prior to operation to no treatment (n = 14), treatment with one capsule t.d.s. (n = 12) or two capsules t.d.s. (n = 11) of a terpene mixture (Rowachol). Patients with pigment stones (n = 7) or no biliary tract disease (n = 5) were also studied. Lipid composition, presence of cholesterol monohydrate crystals, and nucleation time were determined in gallbladder bile aspirated during surgery. Cholesterol saturation was similar in the different groups. Crystals were present in all cholesterol gallstone patients without treatment and in none of the controls. In one of the patients treated with one capsule and four of the patients treated with two capsules crystals could not be detected. The terpenes prolonged nucleation time from 2.8 to 5.8 days (one capsule; P less than 0.05) and to 9.5 days (two capsules; P less than 0.001), respectively; but nucleation did not occur in seven controls. Although the mechanism by which the terpene mixture inhibits the formation of cholesterol crystals in bile was not determined, the findings suggest that the terpene mixture might be a useful agent for a clinical trial to test whether they will prevent recurrence of gallstones after medical dissolution.