Relationships between reproductive factors and risk of cancer of the uterine corpus were investigated in a prospective study of 62,079 women in Norway. A total of 420 cases were diagnosed during follow-up, from 1961 through 1980. The risk of endometrial carcinoma decreased significantly with increasing parity as well as with increasing age at first and last birth. The estimated odds ratio for women with 5 or more births versus uniparous was 0.60 in analyses with adjustment for age and residence characteristics. For first birth at age greater than or equal to 35 versus less than or equal to 19 the odds ratio was 0.48, and for last birth at age greater than or equal to 40 versus less than or equal to 24 we found an odds ratio of 0.45, in analyses with additional adjustment for parity. Significant associations were also found with age at menarche and menopause, the highest risks being observed for women with early menarche or late menopause. The different reproductive variables seemed to affect the risk of sarcomas of the uterine corpus and the risk of endometrial carcinomas in a similar way.