The results of a prospective study on oral contraceptive use and breast disease in northern Alberta are presented. The study groups comprised all women aged 30 to 49 examined in diagnostic breast clinics at the Cross Cancer Institute between 1971 and 1974. Three hundred and one patients had breast cancer, 692 had a subsequent biopsy for a benign breast condition, and 548 had no subsequent biopsy. A tendency for an increased relative risk (RR) of breast cancer in women taking oral contraceptives for periods of 1 to 5 years was evident, with relative risk decreased or unaffected in users of less than 12 months (RR = 0.6) or more than 5 years (RR = 1.0). A slightly increased risk was apparent in patients using oral contraceptives within a year prior to attendance at the clinic (recent users); this increase was emphasized when recent users with a prior biopsy for benign breast disease were analyzed alone (RR = 5.0). In women with a prior breast biopsy, use of oral contraceptives for more than 5 years increased risk of breast cancer nine-fold. Former users who had taken oral contraceptives for less than a year showed a significant reduction in breast cancer risk (RR = 0.3). The risk of benign breast disease was also reduced in former users (RR = 0.6) as well as in long-term users (RR = 0.5).