We report data on 43 patients with polyarteritis affecting the kidneys. The majority (41 patients) had renal histological evidence of microscopic polyarteritis. Although most patients (30 of 43) had significant renal impairment at the time of diagnosis (serum creatinine greater than 250 mumol/l) only five had a symptom, macroscopic haematuria, that directed attention to the kidneys. In the majority of patients in whom data was available there was rapid deterioration in renal function between presentation and diagnosis. Renal function at diagnosis was worse in patients aged over 50 of whom 20 out of 29 had a serum creatinine greater than 500 mumol/l compared with only four of 14 patients aged less than 50. The prognosis was worse in patients over 50 (41 per cent died), in patients with a serum creatinine higher than 500 mumol/l (54 per cent died) and in patients treated with intravenous methylprednisolone, (four also had intravenous cyclophosphamide) (38 per cent died). The major cause of death was sepsis and the actuarial one-year survival was 62 per cent. These results suggest that our approach to treatment should be modified towards lessening immunosuppression in older patients and in patients with renal failure at diagnosis.