Evaluation and therapy of breakthrough bleeding in women using a triphasic oral contraceptive. This study was designed to investigate the incidence and pattern of breakthrough bleeding (BTB) in 1,259 women who were prescribed for the first time a triphasic oral contraceptive (OC, 7-7-7) and to evaluate a hypothesis of management for BTB persisting after three cycles. The new users were compared with a control group of 696 women who had used various OCs for at least 6 months. The incidence of BTB in the control group was 16.8% and in the new users was 24.9%, 17.5%, and 15.3% in the first 3 months, respectively. Breakthrough bleeding occurred late in the 7-7-7 package in 58% and early or midway through the package in 17% and 25%, respectively. We hypothesized that late-package BTB would improve if the patient was switched to a monophasic pill similar to the relatively estrogenic formulation of the beginning of the package and vice versa for early or midpackage BTB. Seventy women with BTB at 3 months were randomly given 0.5/35 or 1/35 for a further 3 months. Breakthrough bleeding was more likely (P less than 0.05) to improve in women switched to 1/35 compared with 0.5/35 regardless of where in the package BTB occurred. 