We prospectively studied and compared clinical features, treatment, course of illness, and long-term morbidity and mortality rates for Wegener granulomatosis in 23 childhood-onset patients with those of 135 adult-onset patients who were studied concurrently. Treatment was usually provided with glucocorticoids and cyclophosphamide. The mean follow-up period was 8.7 years for childhood-onset and 7.6 years for adult-onset Wegener granulomatosis. Most aspects of Wegener granulomatosis were similar in childhood-onset and adult-onset patients. Permanent morbidity from disease occurred in 86% of both groups. However, some features were significantly different. Wegener granulomatosis in childhood-onset patients was complicated five times more often by subglottic stenosis and twice as often by nasal deformity. Treatment-related permanent morbidity occurred in 22% of childhood-onset patients and 45% of adult-onset patients. After similar periods of cyclophosphamide therapy and follow-up, cyclophosphamide-related malignancies were less likely (0% vs 11%) to have developed in childhood-onset patients. Although 89% of patients treated with glucocorticoids and cyclophosphamide had remission, prolonged delay in achieving remission and relapses led in both patient groups to freedom from active disease for approximately 50% of the total patient-years. As a result, morbidity was substantial and has led to comparative studies of alternative therapies.