Long-term study of 21 PNH patients revealed an unexpectedly high incidence of functional and anatomic renal abnormalities. Most patients demonstrated varying degrees of hematuria and proteinuria distinct from hemoglobinuria. Evaluation of renal function revealed hyposthenuria, abnormal tubular function, and declining creatinine clearance. Radiologically these patients had enlarged kidneys, cortical infarcts, cortical thinning, and papillary necrosis which were confirmed by autopsy studies. Hypertension developed in eight patients. Urinary tract infection was uncommon. The renal findings bear striking similarity to those of sickle cell anemia. Contrary to the usual opinion, out studies clearly showed evidence of widespread renal pathology in PNH most likely due to repeated microvascular thrombosis similar to the venous thrombosis involving other organs in this disorder.