We prospectively studied trimethoprim/sulfamethoxazole (T/S) in inducing remission in 'initial phase' Wegener's granulomatosis (WG), and in sustaining remission in generalized WG, in 72 patients in various disease stages. Nineteen patients with initial phase WG received T/S (2 x 960 mg/day). Another 24 patients with generalized WG received the same dose of T/S (group A) and were compared with 21 patients receiving no further treatment after standard therapy (group B). Eight patients were given T/S plus low-dose prednisone (group C). Eleven of 19 patients (58%) with initial phase WG achieved complete or partial remission lasting a median 43 months (range 6-88 months). Of the remaining eight (42%), five showed local disease progression, and three developed generalized WG. In group A (T/S alone, generalized WG), 10/24 (42%) suffered a relapse after a median 13 months (range 4-58 months). In group B (generalized WG, no further treatment) 29% of patients relapsed after a median 22.5 months (range 18-26 months). All eight patients treated with T/S plus low-dose prednisone (group C) suffered serious relapse after 2-24 months. T/S induced long-term remission in > 50% of patients with initial phase WG; however, neither T/S alone nor T/S plus low-dose prednisone sustained remission in generalized WG.