Introduction Transsexual people receive cross‐sex hormones as part of their treatment, potentially inducing hormone‐sensitive malignancies.

Aim To examine the occurrence of breast cancer in a large cohort of Dutch male and female transsexual persons, also evaluating whether the epidemiology accords with the natal sex or the new sex.

Main Outcome Measure Number of people with breast cancer between 1975 and 2011.

Methods We researched the occurrence of breast cancer among transsexual persons 18–80 years with an exposure to cross‐sex hormones between 5 to >30 years. Our study included 2,307 male‐to‐female (MtF) transsexual persons undergoing androgen deprivation and estrogen administration (52,370 person‐years of exposure), and 795 female‐to‐male (FtM) subjects receiving testosterone (15,974 total years of exposure).

Results Among MtF individuals one case was encountered, as well as a probable but not proven second case. The estimated rate of 4.1 per 100,000 person‐years (95% confidence interval [CI]: 0.8–13.0) was lower than expected if these two cases are regarded as female breast cancer, but within expectations if viewed as male breast cancer. In FtM subjects, who were younger and had shorter exposure to cross‐sex hormones compared with the MtF group, one breast cancer case occurred. This translated into a rate of 5.9 per 100,000 person‐years (95% CI: 0.5–27.4), again lower than expected for female breast cancer but within expected norms for male breast cancer.