There are marked variations in the risk of hormone-dependent cancers between males and females, and these are likely to reflect sex differences in endogenous hormone profiles. The authors examined sex differences in the risk of hormone-dependent cancers of sex-shared sites by using data from the England and Wales national cancer registry for 1962-1984. Both breast and thyroid cancers showed marked excesses in risk for women, but the female: male ratio peaked around menopause for breast cancer and a puberty for thyroid cancer, suggesting that although female sex hormones may influence the risk of these two cancers, the mechanisms involved are probably different. In the descending colon, the risk of cancer was greater in females than in males at ages under 60 years, but greatest in males at ages above this, whereas in the ascending colon there were no age-specific differences in risk between the sexes. This is consistent with the hypothesis that female reproductive events may decrease a woman's risk of cancer in the descending but not in the ascending colon. Sex differences in bone cancer risk at puberty, particularly for osteosarcomas and Ewing's sarcomas, paralleled known sex differences in skeletal growth; there was a peak in age-specific rates earlier and lower in girls than in boys. Rhabdomyosarcoma, a soft tissue cancer, also showed a rise in risk at puberty with age differences between boys and girls that correlated with sex differences in muscle growth patterns; this suggests that its etiology may be hormonally related as well.