The belief that oestrogens are involved in the pathogenesis both of testicular cancer in young men and of cancers of the endometrium and female breast has become widespread. In a search for possible hormonal links between these cancers, we investigated the cancer pattern in a cohort of women who had given birth to sons who developed testicular cancer. particular focus was given to oestrogen-related cancers. The present retrospective population-based cohort study is based on data from the Danish Cancer Registry. Mothers of 2,204 testicular-cancer patients were followed for the occurrence of cancer over a total of 70,063 person years. The ratio of observed cancers in the cohort over the expected numbers based on cancer incidence in the underlying female population served as measure of the relative risk (RR). The RR of developing breast cancer among mothers of testicular-cancer patients was 0.8 (95% confidence interval 0.6-1.1), the relative risk of endometrial cancer 0.6 (0.3-1.0) and of ovarian cancer 1.0 (0.6-1.6). Mothers of testicular-cancer patients are not at increased risk of developing oestrogen-related cancers.