A patient with ocular hypertension was treated with acetazolamide. Acute renal failure developed rapidly and renal biopsy showed mild tubular lesions and crystal formation in a tubular lumen. By immunofluorescence studies with a monoclonal antibody, Tamm-Horsfall protein, normally absent from the proximal segments of the nephron, was detected in most glomeruli. This strongly suggests that tubular obstruction plays a major part in some cases of acetazolamide-induced acute renal failure.