Fifteen patients who developed cancer of the endometrium 10 or more years after pelvic irradiation for carcinoma were selected for study from a group of 64 cases of postirradiation malignant pelvic tumors diagnosed during a 48-year span. The average interval between radiotherapy and diagnosis of the subsequent endometrial cancer was 17.2 years. Irradiation initially had been done for squamous cell carcinoma of the cervix in 13 cases (87%) and for ovarian tumors in two instances. Almost all patients had received megavoltage external radiation combined with radium implants. Two-thirds of the tumors were adenocarcinomas and one-third were carcinosarcomas (either homologous or heterologous). Although the risk of second primary malignant tumors following therapeutic irradiation for pelvic tumors probably is very low, the emergence of new genital tract cancers in long-term survivors must be anticipated, regardless of whether the postirradiation cancers are spontaneous or radiation-induced.