An autopsy series on the patterns of occurrence of second primary non-mammary malignancies (SPNM) in breast cancer patients is presented. SPNM occurred in 11.4% of breast cancer patients at a mean interval of 83.9 months. Seventy-five per cent of them occurred by the first decade following breast cancer. The most common system of SPNM occurrence was the female genital system, followed by the gastrointestinal system. The single organs most commonly involved were the colon and rectum, ovary and uterine cervix. There was no evident association between administration of radiotherapy or chemotherapy for the breast cancer and the risk for SPNM development except in the case of post-radiation sarcomas at or near the site of post-mastectomy radiotherapy. Death was caused by the breast cancer in 28.7% of patients and by the SPNM in 53.5%. In 10.6% of the cases the SPNM was clinically interpreted as metastatic breast cancer until autopsy.