Renal biopsies were performed on 91 out of 99 patients with acute intrinsic renal failure. In all these 91 cases a clinical diagnosis was suggested by an experienced nephrologist unaware of the renal biopsy findings. Most diagnoses agreed with the histopathological findings, but there were important exceptions. On clinical grounds, acute tubulointerstitial renal disease was diagnosed correctly in 77% and acute glomerulonephritis in 56%. In 15% of the patients the clinical data did not permit any certain diagnostic suggestion. In about 20% the biopsy finding definitely influenced the choice of medical treatment. In those 15 patients who needed dialysis treatment, the biopsy findings helped in determining whether this need was temporary or permanent. Our results show that renal biopsy is valuable in settling the diagnosis, in determining the prognosis, and in planning the treatment of acute intrinsic renal failure.