Rapidly progressive renal insufficiency is rare in patients with classic polyarteritis nodosa (cPAN). We describe two cases of cPAN who presented with rapid and progressive deterioration of renal function. Renal biopsies showed severe necrotizing vasculitis in medium-sized arteries but no changes in glomeruli. Combination therapy of corticosteroid and cyclophosphamide resulted in marked improvement of clinical symptoms, amelioration of renal function and lack of subsequent relapse of vasculitis. These findings suggest good prognosis of patients with cPAN who show rapid and progressive deterioration of renal function who respond to immunosuppressive therapy.