Severe acute deterioration of renal function, which occurred during the episode of massive gross hematuria, is described in two patients with IgA nephropathy (IgAN). The renal failure abated gradually after dialytic support. Renal pathology in each case revealed focal proliferative glomerulonephritis with less than 14% of the glomeruli affected by crescents or sclerosis. In contrast to the mild glomerular lesions, acute tubular cell injury in relation to phagocytosis of erythrocytes or pigments and/or tubular obstruction was prominent. We think our observations support the previous reports that tubular lesions in relation to heavy glomerular bleeding may be a cause of severe acute renal failure in patients with IgAN.