There is great debate as to whether the benefit gained from the knowledge of renal histology outweighs the risk to the patient from the biopsy procedure. We conducted a prospective study of 276 native renal biopsies performed on 266 patients from a single centre in 1991 to assess the effect of the knowledge of renal histology on patient management. Biopsies were performed under ultrasound guidance using the Trucut biopsy needle. The indications for biopsy were: non-nephrotic proteinuria alone (25), haematuria and proteinuria (28), nephrotic range proteinuria (28), acute renal failure (31), haematuria alone (36), and chronic renal failure (128). Two hundred and sixty-three biopsies were successful. The mean number of glomeruli obtained was 23, range 0-115. Eight patients developed macroscopic haematuria of which two required blood transfusion. The result of the biopsy altered management in 24/28 (86%) of cases of nephrotic range proteinuria, 22/31 (71%) of cases of acute renal failure, 58/128 (45%) of cases of chronic renal failure, 9/28 (32%) of cases with haematuria and proteinuria, 3/25 (12%) of cases with non-nephrotic proteinuria alone, and 1/36 (3%) of cases with haematuria alone. management was altered in 42% of cases overall. These data suggest that knowledge of renal histology is essential in the management of patients with renal disease.