The incidence of endometrial carcinomas as well as the life-spans of 5 groups of female BD II/Han inbred rats were compared. Groups compared were control (CON), ovariectomized (OV), retired breeder (RB), germfree rats (GF), and rats fed a purified diet (PD). Carcinomas, for the most part adenocarcinomas, occurred in 90% of the CON and PD rats and approximately 60% of the RB and GF rats. Carcinomas did not develop in OV rats. The mean life-span of the CON and PD rats was 22 months. The RB and GF rats reached, respectively, 32 and 30 months; and the OV rats, 39 months. An essential influence of ovarian hormones on tumorigenesis was shown. Alimentary carcinogens and phytoestrogens, as well as chronic irritations of the endometrium due to bacteriologic inflammations, could be excluded as tumor initiators. Hormonal dysregulation in the aging BD II/Han rats was discussed as a possible cause of an inappropriate stimulation of the endometrium, promoting a previously transformed tumor cell or initiating an autonomous tumor growth. Such evidence supports the use of the BD II/Han rat as a suitable model for the study of hormonal effects on the tumorigenesis of endometrial carcinomas in women.