The effectiveness of specific antibiotic treatment in severe leptospirosis is still under debate. As part of a prospective study designed to evaluate renal function recovery after leptospirosis acute renal failure (ARF) (ARF was defined as Pcr > or =1.5 mg/dL), the clinical evolutions of 16 treated patients (T) were compared to those of 18 untreated patients (nT). Treatment or non-treatment was the option of each patient's attending infectologist. The penicillin treatment was always with 6 million IU/day for 8 days. No difference was found between the two groups in terms of age, gender, number of days from onset of symptoms to hospital admission, or results of laboratory tests performed upon admission and during hospitalization, but proteinuria was higher in the treated group. There were no significant difference in the other parameters employed to evaluate patients' clinical evolution as: length of hospital stay, days of fever, days to normalization of renal function, days to total bilirubins normalized or reached 1/3 of maximum value and days to normalization of platelet counts. Dialytic treatment indication and mortality were similar between group T and nT. In conclusion, penicillin therapy did not provide better clinical outcome in patients with leptospirosis and ARF.