Tamoxifen (TAM), a nonsteroidal antiestrogen, is used for pre- and postmenopausal patients with breast cancer. Recent reports suggest that TAM may cause endometrial neoplasia. This study is designed to evaluate the oncogenic potential of low-dose TAM on the endometrium. Initially, endometrial screening of patients with breast cancer who had received TAM therapy for at least 12 months was conducted. Seventy patients were interviewed and office endometrial biopsies were obtained from thirty-eight patients. Seven (18%) had hyperplastic changes, ranging from simple hyperplasia through complex hyperplasia with atypia. The following prospective study was conducted: after breast surgery and prior to initiation of TAM therapy, an office endometrial sampling was obtained as a control. After initiation of TAM therapy, biopsies were repeated every 4 to 6 months as long as the patients remained asymptomatic. Nineteen patients were interviewed. Twelve patients were biopsied and followed from 3 to 15 months. One patient refused additional biopsies. Eleven patients had repeat biopsies after initiation of TAM. New hyperplastic changes were found in 3/11 (27%) patients. The preliminary results of this study (although with a small number of patients) indicate that TAM may have some neoplastic effect on the endometrium of postmenopausal patients with breast cancer. This study is still in progress. Additional prospective studies are warranted before a significant correlation between TAM and endometrial neoplasia is confirmed.