The clinical manifestations, treatment and survival of 64 patients with polyarteritis nodosa seen from 1955 to 1977 were evaluated. In general, the patients had multisystem involvement. No patient with cutaneous vasculitis alone was accepted into the study. The clinical diagnosis was confirmed by biopsy in 34 patients, by autopsy in 13 and by angiography in 10. The patients were treated at the discretion of the physicians responsible for their care. Eight of the 64 patients received only supportive therapy (group 1), 34 received corticosteroids alone (group 2), and 22 received both corticosteroids and an immunosuppressive agent (group 3). Five patients in group 2 and one patient in group 3 were excluded from survival studies because of insufficient length of therapy. Patients in the three treatment groups were very similar with respect to 18 clinical and laboratory variables. Median survival times for the three groups were three months, 63 months and 149 months, respectively; 5 year survival rates were 12 per cent, 53 per cent and 80 per cent (p less than 0.05). Despite difficulty in precisely defining polyarteritis nodosa, the data suggest a better prognosis for treated patients than has previously been appreciated, with improvement in outcome when an immunosuppressive agent is added to corticosteroid therapy.