During the four-year period 1981-1984, 250 patients with severe acute renal failure were treated at one centre. There were seven obstetric cases (2.8 per cent) 118 'surgical' cases (47.2 per cent) and 125 medical cases (50 per cent). This is a different pattern from that seen in the majority of earlier reports. In 60 of the 125 medical patients the aetiology of the acute renal failure could only be determined by renal biopsy. This series suggests that with changing medical practice (particularly the improvement in resuscitation) and an ageing population, the pattern of causes of acute renal failure is altering. It also highlights the value of renal histology as a guide to diagnosis and treatment in patients with unexplained acute renal failure.