The purpose of this study was to examine drug interactions between boceprevir, a hepatitis C virus NS3/4A protease inhibitor, and a combined oral contraceptive containing ethinyl estradiol (EE) and norethindrone (NE). A single-center, open-label study was conducted in 20 healthy female volunteers. In three consecutive 28-day treatment periods, subjects received EE/NE (0.035 mg/1 mg; 21 days on, 7 days off). During period 3, subjects also received boceprevir (800 mg three times daily) for 28 days. Coadministration of boceprevir with EE/NE did not affect NE AUC0–24 but slightly reduced NE C max. Geometric mean ratios (GMRs) for NE AUC0–24 and C max with EE/NE alone and EE/NE plus boceprevir were 0.96 (90 % confidence interval (CI), 0.87–1.06) and 0.83 (90 % CI, 0.76–0.90). Coadministration of boceprevir with EE/NE reduced EE AUC0–24 and C max by 26 and 21 %, with GMRs of 0.74 (90 % CI, 0.68–0.80) and 0.79 (90 % CI, 0.75–0.84). Boceprevir had no effect on mid-cycle luteinizing hormone (LH), follicle-stimulating hormone (FSH), or sex hormone-binding globulin levels, and progesterone concentrations remained