We measured the metabolites of NO [nitrite (NO2-) and nitrate (NO3-)] in urine from Ethiopian patients suffering from leprosy. The urinary level of NO2-/NO3- in a group of healthy Ethiopians was 1020 +/- 471 microM (n = 22). Leprosy patients in reversal reaction had significantly higher levels of NO2-/NO3- (1817 +/- 492 microM, p < 0.001, n = 12) than both the control group and leprosy patients who were not in reversal reaction (1079 +/- 446 microM, n = 12). We conclude that the reversal reaction in leprosy in associated with increased urinary levels of nitric oxide metabolites.