The clinical course of 40 patients with polyarteritis was reviewed to determine prognostic factors and response to treatment. The first three months were the most critical to survival. Survivorship was 57 per cent at five years. Older age of onset, involvement of skeletal muscle and presence of peripheral neuropathy weighted against a satisfactory outcome. Cutaneous vasculitis was associated with a more benign course. Myocardial disease, central nervous system involvement, or hypertension were not invariably poor prognostic factors. Muscle biopsies, even in the absence of clinical involvement, were a useful diagnostic procedure, and renal angiograms were found to be a valuable alternative to renal biopsy. An unequivocal distinction on clinical and histopathologic criteria could not be made among polyarteritis nodosa, hypersentitivity angiitis, and allergic granulomatosis. Australia antigenemia occurred in six per cent of patients. Although evaluation of therapy was difficult, data from this study did not show a superiority of high vs. low dosage of corticosteroids in suppressing active disease.