In view of the carcinogenic potential and widespread use of conjugated estrogens (CEs), this study evaluated patterns and determinants of oral ambulatory CE use. Among 1273 women 30 to 69 years of age interviewed in Greater-Boston hospitals, there were 161 (13%) ever-users of CEs; 88 (7%) had used the drug within the preceding year. The median duration of use was three years. Prominent determinants of use were age, menopause and history of menopausal symptoms. Physician prescription survey data from an independent source suggest that CE use has been common throughout the United States, and may have declined after the reports linking CEs to endometrial cancer were published in 1975. If CE use indeed increases the risk of endometrial cancer some fivefold, an important public health problem exists.