The recent increase in hepatocellular neoplasms in women of child-bearing age taking oral contraceptives and in individuals of both sexes taking anabolic androgenic steroids is a phenomenon that requires urgent attention. Although most of the lesions are benign, reports of carcinomas in a few of these individuals and our own observation of an adenoma that shows borderline malignant transformation suggest that some of the adenomas are premalignant lesions. The pathology of our own cases and those reported in the literature are reviewed, and hypotheses are offered for a carcinogenic role of synthetic steroid sex hormones. These speculations are based on known toxic and metabolic actions of the drugs and are amenable to experimentation.