Acute renal insufficiency is a common problem, yet one that is frequently reversible with proper diagnosis and treatment. Although it has been argued that a renal biopsy is not needed for diagnosis in most cases of acute renal failure in the elderly, other studies have shown frequent disagreements between clinical and renal biopsy diagnoses in such cases. To investigate the causes of acute renal insufficiency in patients aged at least 60 years who underwent a renal biopsy and possible correlations between biopsy findings and renal survival, we first identified all native renal biopsy specimens from patients aged 60 years or older processed at The University of Chicago Medical Center (Chicago, IL) from 1991 through 1998 and reviewed the clinical records to determine the indication for the biopsy. We then reviewed again the records of those patients who underwent biopsy because of acute renal insufficiency, recorded the primary renal biopsy diagnosis in each of these cases, and obtained follow-up information for patients who underwent biopsy before July 1996. During the study period, 1,065 of 4,264 biopsy specimens (25.0%) received were obtained from patients aged 60 years or older, and acute renal insufficiency was the indication for biopsy in 259 of these patients (24.3%). The most frequent primary diagnoses on these latter biopsy specimens were pauci-immune crescentic glomerulonephritis (GN) with or without arteritis, 31.2% of biopsy specimens; acute interstitial nephritis, 18.6%; acute tubular necrosis (ATN) with nephrotic syndrome, 7.5%; atheroemboli, 7.1%; ATN alone, 6.7%; light chain cast nephropathy (LCCN), 5.9%; postinfectious GN, 5.5%; anti-glomerular basement membrane antibody nephritis, 4.0%; and immunoglobulin A (IgA) nephropathy and/or Henoch-Schönlein nephritis, 3.6%. Eight biopsy specimens (3.2%) showed only benign nephrosclerosis without an apparent cause of acute renal insufficiency, and another six specimens were inadequate. The renal biopsy diagnosis was in agreement with the prebiopsy clinical diagnosis (or differential diagnosis) in 107 of the 161 cases (67%) in which such information was provided. The distribution of diagnoses was similar in patients in the age groups of 60 to 69, 70 to 79, and 80 years or older, although younger age correlated significantly with improved renal and patient survival. The relative risk for progression to end-stage renal disease (ESRD) also increased according to diagnostic categories: LCCN (greatest risk) > GN other than pauci-immune > atheroemboli congruent with pauci-immune crescentic GN > tubulointerstitial diseases other than LCCN (the latter category including ATN with nephrotic syndrome). Development of ESRD correlated significantly with decreased patient survival. In summary, renal biopsy in patients aged 60 years or older with acute renal insufficiency uncovered the cause in greater than 90% of the cases and provided clinically useful information with respect to expectation for renal survival and potential treatment options.