The appearance of renal failure during episodes of macroscopic hematuria (EMH) in IgA nephropathy (IgAN) has been described as very unusual. The results of a prospective investigation on the effect of EMH on renal function in IgAN are presented. During a 3-year period, 29 episodes of EMH occurring in 21 patients with IgAN have been studied. A derangement of renal function (increase of serum creatinine by more than 0.5 mg/dl) was observed in 11 episodes (37.9%) with peak creatinine values ranging from 1.2 to 6.7 mg/dl. The worsening of renal function was accompanied by a longer duration of EMH (4.8 +/- 1.3 vs. 3.5 +/- 1.5 days; P less than 0.05) but not by arterial hypertension or edema. A complete recovery of renal function was observed in every patient 1 to 2 months after the start of EMH. The histological survey disclosed that the decrease of renal function correlated closely with the presence of red blood cell casts in as much as 50% of the tubular lumen and with findings of tubular necrosis. We conclude that a worsening of renal function can be observed frequently during the EMH. Tubular damage and obstruction by red blood cell casts may play a significant role in the pathogenesis of this complication.