The relationship between Quetelet's index and subsequent risk for cancer of endocrine target organs was studied in a cohort of 47,003 women, examined for height and weight in the years 1963-65, and followed up in the Swedish Cancer Register until 1987. High Quetelet's index was associated with a decreased risk for breast cancer among women less than 55 years of age at risk, while a high Quetelet's index predicted an increased risk among older women. Among women > or = 55 years of age, the excess relative risk for breast cancer associated with high Quetelet's index declined significantly during the follow-up period. Cancer of the ovaries and the uterine cervix were not significantly related to Quetelet's index in any age group. In women > or = 55 years of age, the relative risk for cancer of the uterine corpus associated to Quetelet's index was higher than that for breast cancer, and this association persisted during the entire follow-up period of more than 20 years. In spite of the fact that endometrial cancer is less common than breast cancer, because of the stronger relation between overweight and endometrial cancer, more endometrial cancer would be attributable to obesity than breast cancer.