We have described 34 patients with microscopic polyarteritis, all of whom had clinical evidence of a systemic small vessel vasculitis predominantly affecting the skin and musculoskeletal systems accompanied by a focal necrotising glomerulonephritis with renal impairment. Thirty three patients received immunosuppressive treatment, comprising prednisolone, azathioprine, cyclophosphamide and plasma exchange in various combinations. The five-year actuarial patient and kidney survival rates were 65 and 55 per cent respectively, suggesting that aggressive immunosuppressive therapy in these patients is beneficial.