Macroscopic haematuria is common in IgA nephropathy, but its significance and influence on prognosis remains uncertain. We compared the clinical and pathological features of 11 adult patients with primary IgA nephropathy who had had a renal biopsy during or shortly after a bleeding episode. Six patients developed transient acute renal failure (ARF) (group 1) and five did not (group 2). Patients of group 1 had a higher percentage of tubular red-blood-cell (RBC) casts (P < 0.05) and of glomerular crescents (P < 0.001). However, crescents were focal and involved less than 50% of glomeruli. Acute tubular necrosis was only present in patients of group 1, and ARF was attributed to the acute tubular changes rather than to the glomerular lesions. Despite a prolonged duration of ARF (mean: 38 days), further outcome did not differ in patients of both groups. We suggest that acute tubular damage and/or tubular obstruction by RBC casts should be considered in any patient who develops ARF soon after a haematuric episode.