Clinical and laboratory features of 31 children with a diagnosis of polyarteritis nodosa were evaluated retrospectively. All the patients had musculoskeletal involvement, renal involvement, or both during the course of the disease. We have defined involvement of these two systems as the major diagnostic criteria in polyarteritis nodosa. Ten additional minor criteria were defined: (1) cutaneous findings, (2) gastrointestinal involvement, (3) peripheral neuropathy, (4) central nervous system involvement, (5) hypertension, (6) cardiac involvement, (7) lung involvement, (8) constitutional symptoms, (9) presence of acute-phase reactants, and (10) presence of hepatitis B surface antigen. We propose that the presence of five of these criteria, including at least one major criterion, is highly suggestive of polyarteritis nodosa; such a combination was present in 97% of our patients. Fourteen of the patients were treated with corticosteroids alone and 14 were treated with a combination of steroids plus cyclophosphamide or azathioprine. At the last follow-up examination six patients in the steroid group and nine in the combination group were considered to have complete remission of disease or inactive disease with persisting symptoms in an organ system. The overall mortality rate was 16%; renal involvement had the greatest adverse effect on outcome. We suggest that in patients with five of the 12 diagnostic criteria, especially those with renal involvement, therapy should be initiated promptly while diagnostic procedures are being carried out.