The incidence of primary breast cancer is known to be increased in patients with previous endometrial or ovarian cancer, but the behavior of the breast cancer and the ultimate outlook for such patients is unknown. To provide data concerning these two questions, a group of 123 patients treated for breast cancer alone (Group I) served as a control for comparison with ten patients who had endometrial cancer (Group 11) and six patients who had ovarian cancer (Group 111) prior to diagnosis of breast cancer. The interval between the diagnosis of endometrial cancer and breast cancer averaged 4.6 yr, and between ovarian cancer and breast cancer, 5.4 yr. Age at diagnosis of breast cancer in Groups I, 11, and 111 was 56, 66 (P = 0.05) and 56 yr, respectively. The incidence of patients with stage I, 11, and 111 breast cancer was similar in Groups I, 11, and 111 as was tumor size and number of metastatic nodes in each stage in each of the three groups. Average duration of follow-up after diagnosis of breast cancer was 3, 3.1, and 2.8 yr in Groups I, 11, and 111, respectively, with 70%, 60%, and 33 % of patients living and free of either breast cancer, endometrial cancer, or ovarian cancer. Two of four deaths in Group I1 were due to endometrial cancer, and all four deaths in Group I11 were due to ovarian cancer. The remaining two deaths or recurrences in Group I1 were due to breast cancer. We conclude.that 1) the behavior of breast cancer is similar in patients with or without previous endometrial or ovarian cancer; 2) breast cancer develops at an older age in patients with previous endometrial cancer than in patients with or without previous ovarian cancer and 3) death or recurrent cancer in patients with breast cancer and previous ovarian cancer is due to ovarian cancer, whereas, death or recurrent cancer in patients with breast cancer and previous endometrial cancer is due equally to breast cancer and endometrial cancer.