Fifty-three patients with polyarteritis who were followed up for at least 2 years were defined clinically and studied retrospectively to determine the influence of clinical factors and treatment on the prognosis. There was a spectrum of severity of disease, and the 5-year survival in the group was 55%. A small number of patients had evidence of ongoing immune-complex disease, as indicated by the presence of cryoglobulins or hepatitis Bs antigen or by diminished serum complement. These markers were not associated with distinct clinical features and did not influence prognosis. Organ involvement that most adversely affected prognosis was that of the gut and the kidneys. Six of 8 patients with bowel infarction or serious gastrointestinal bleeding died, and 6 of 10 patients with renal insufficiency died. Hypertension and peripheral neuropathy did not influence the prognosis. Thirty-six patients were treated with corticosteroids alone and 14 with a combination of corticosteroids and cytotoxic agents (3 received no treatment); the outcome was the same in both groups. Twenty-two in the steroid-alone group and six in the combination group were alive when last seen. Early deaths were usually due to complications directly related to the vasculitis, and late deaths were often due to cerebrovascular or cardiovascular complications. At the last follow-up, 18 patients were in remission, and 13 had inactive vasculitic disease and were on maintenance treatment.