During the past six years an increasing number of liver-cell adenomas in women were diagnosed, and we undertook a study of possible etiologic factors, including use of oral contraceptives, in 42 women. There was a significant difference between cases and matched controls in mean months of oral-contraceptive use: 73.4 as compared to 36.2 (P less than 0.001). The women with adenomas took mestranol-containing pills much more commonly than the controls (P less than 0.0001). Hemorrhage into the tumor was often associated with the onset of menstruation in women taking oral contraceptives. Long-term use of oral contraceptives, especially those containing a high total dose of steroids or mestranol as the synthetic estrogen, should be balanced against risks of side effects of the drugs.