The purpose of this treatment package design study was to investigate the differential efficacy of cognitive therapy or exposure in vivo with response prevention for obsessive compulsive disorder (OCD) versus the sequential combination with fluvoxamine. Patients with OCD (N = 117) were randomized to one of the following five conditions: a) cognitive therapy for weeks 1 to 16, b) exposure in vivo with response prevention for weeks 1 to 16, c) fluvoxamine for weeks 1 to 16 plus cognitive therapy in weeks 9 to 16, d) fluvoxamine for weeks 1 to 16 plus exposure in vivo with response prevention in weeks 9 to 16, or e) waiting list control condition for weeks 1 to 8 only. Assessments took place before treatment (pretest) and after 8 (midtest), and 16 weeks (posttest). In the first 8 weeks, six treatment sessions were delivered. During weeks 9 to 16, another 10 sessions were given. Thirty-one patients dropped out. Outcome was assessed by patient-, therapist- and assessor-ratings of the Anxiety Discomfort Scale, the Yale-Brown Obsessive Compulsive Scale, and the Padua Inventory-Revised. In contrast with the four treatments, after 8 weeks the waiting list control condition did not result in a significant decrease of symptoms. After 16 weeks of treatment, all four treatment packages were effective on these OCD ratings, but they did not differ among each other in effectiveness. In OCD, the sequential combination of fluvoxamine with cognitive therapy or exposure in vivo with response prevention is not superior to either cognitive therapy or exposure in vivo alone.