The daily administration of fish oil, containing 1.5 g omega-3-fatty acids causes a significant decrease in nucleation time (12.1 +/-7.3 vs. 2.0 +/- 1.2 days, p less than 0.001), as well as in biliary cholesterol saturation and biliary cholesterol composition in 13 healthy subjects. The nucleation time, cholesterol saturation index and biliary lipids showed no significant differences in 11 cholesterol gallstone patients after a six-week treatment period with omega-3-fatty acids. Taurocholate percentage increased significantly in the gallstone group (10.4 +/- 3.9 vs 13.5 +/- 1.7%, p less than 0.05). The levels of the other bile acids remained unchanged during the treatment period. Therefore, the incidence of gallstones might be increased after treatment with fish oil, containing omega-3-fatty acids.