BACKGROUND Respiratory tract infections may trigger relapses in patients with Wegener's granulomatosis in remission. Uncontrolled data have suggested that treatment with trimethoprim-sulfamethoxazole (co-trimoxazole) may be beneficial. METHODS We conducted a prospective, randomized, placebo-controlled study of the efficacy of co-trimoxazole (800 mg of sulfamethoxazole and 160 mg of trimethoprim) given twice daily for 24 months in preventing relapses in patients with Wegener's granulomatosis in remission during or after treatment with cyclophosphamide and prednisolone. Relapses and infections were assessed with predefined criteria based on clinical, laboratory, and histopathological findings. Patients were evaluated at least once every three months for signs of disease activity, compliance with the treatment regimen, side effects of the therapy, and evidence of infections. Titers of serum antineutrophil cytoplasmic antibodies were measured serially. RESULTS Forty-one patients were assigned to receive co-trimoxazole, and 40 to receive placebo. In 8 of the 41 patients in the co-trimoxazole group (20 percent), the drug had to be stopped because of side effects. According to life-table analysis, 82 percent of the patients remained in remission at 24 months, as compared with 60 percent of the patients in the placebo group (relative risk of relapse, 0.40; 95 percent confidence interval, 0.17 to 0.98). There were fewer respiratory tract infections (P = 0.005) and non-respiratory tract infections (P = 0.05) in the co-trimoxazole group than in the placebo group. There were no significant differences in antineutrophil cytoplasmic antibody titers at any time. Proportional-hazards regression analysis identified treatment with co-trimoxazole as an independent factor associated with prolonged disease-free survival and a positive antineutrophil cytoplasmic antibody test at the start of treatment as a risk factor for relapse. CONCLUSIONS Treatment with co-trimoxazole reduces the incidence of relapses in patients with Wegener's granulomatosis in remission.