We report renal biopsy findings in 109 patients with unexplained renal impairment (serum creatinine greater than 0.15 mmol/l) and normal-sized non-obstructed kidneys. The most common histological lesions were interstitial nephritis, rapidly progressive glomerulonephritis and a variety of other types of glomerulonephritis. The groups could not be distinguished by the presence or absence of hypertension, haematuria, proteinuria, or features of systemic disease. However interstitial nephritis was found more frequently in patients presenting with one or none of these features and rapidly progressive glomerulonephritis in patients presenting with three or more. All four patients with none of these features had interstitial lesions. Fifty-two per cent of patients with interstitial nephritis improved and 60 per cent of the patients with rapidly progressive glomerulonephritis who received immunosuppressive treatment improved or remained stable with treatment. The benefits of a biopsy diagnosis were almost wholly confined to these two groups. Complications were recorded in nine patients - prolonged macroscopic haematuria in six and symptomatic perirenal haematomata in three. Six required blood transfusion. One required nephrectomy to control haemorrhage and subsequently died. Percutaneous renal biopsy is not without risk in patients with renal impairment but the benefits of diagnosing interstitial nephritis and rapidly progressive glomerulonephritis outweigh the disadvantages.