We studied the features of acute renal failure (ARF) in elderly patients treated in a hospital, without an intensive care unit, to identify etiological factors and establish adequate preventive measures and treatment. During twelve consecutive months we studied prospectively 99 patients with ARF diagnosed by conventional criteria, an incidence of 1,238 cases per million per year. ARF affected 1.78% of patients admitted to hospital. We analyzed age, sex, serum creatinine, diuresis, etiology, type of ARF, preexisting chronic diseases, treatment, complications and outcome. Preexisting chronic diseases were common, the most frequent being hypertension (54%) and diabetes (39%). Previous treatments for cardiovascular diseases were frequent (angiotensin-renin system blockade 35.4%, diuretics 50.5%). 79% of ARF arose in hospital, 21% outside hospital. ARF was pre-renal in 60%, renal in 31% and post-renal in 9%. 34.7% were caused by volume depletion, 23.4% by low cardiac output and 23.4% by infection. 44.4% of ARF patients had oliguria or anuria latrogenic factors contributed to the ethiology of ARF in 35.3% of patients. Hospital stay was doubled by ARF the presence of ARF and the mortality was 36.4%. The rate was higher in ARF arising in hospital than in ARF acquired before admission. Factors that had a significant influence on the mortality rate were comorbid conditions, oliguroanuria, ARF of renal origin and serum albumin. We conclude that ARF has a high incidence, morbidity and mortality in this elderly population. Volume depletion, associated cardiovascular pathology and pharmacological treatment are important etiological factors in those with ARF outside hospital. Adequate treatment of ARF and avoidance of nephrotoxic medications are necessary in hospital.