Urinary tract infections in the elderly are common, often asymptomatic and usually benign. We report three patients who presented with acute renal failure due to acute pyelonephritis in the absence of clinical findings of infection or urinary tract obstruction. Blood and urine cultures grew Escherichia coli in two of the patients and in two patients renal biopsy confirmed acute pyogenic pyelonephritis. Antimicrobial therapy and haemodialysis led to improvement, though one patient subsequently died from an unrelated cause. We suggest that acute bacterial pyelonephritis should be considered as a cause of acute renal failure in the elderly. Clinical features of infection may be absent despite bacteraemia. Prompt diagnosis and intervention may avoid chronic renal failure in a group that has a less favourable outcome with long-term dialysis.