Niridazole, a nitrothiazole derivative, was tried in twelve patients with American cutaneous leishmaniasis. In all the patients the clinical diagnosis of the disease was confirmed by biopsy of the lesions and in eight also by a Montenegro's positive skin test. Eight patients had active metastatic mucosal lesions and initial skin lesions healed by a period of time more or less prolonged. Two had concomitant cutaneous and mucosal lesions in activity and two active cutaneous lesions only. The duration of the disease varied from two months to thirty tico years. The daily dosage of niridazole was uniformly 25 mg/kg of body weight administered by oral reate, in two daily parcelated doses. The patients were always hospitalized and when postible, by tolerance of the drug, received five courses of treatment of a ten days duration each. intervaled by the supression of the drug by ten days after each course of treatment. This was possible in ten from twelve patients. The treatment was well tolerated by five patients and regulary by four and badly tolerated in one patient due to appearance of hallucitations and mental excitement. The treatment was interrupted in two patients by the occurrence of convulsive seizure with loss of consciousness. The treatment ivas acc.om - panied by high incidence of side-effects. The anthor could not relate the more intense side-effects to bad hepatic functions. Liver function tests, blood cells and biochernical blood studies and urinalysis were carried out in all patients before, during and after treatment and, showed no signifficant changes. Needle biopsy of the liver failed to demonstrate evidence of hepatic damage in all ten patients submitted to the study. The follow-up period, of this therapeutic study were prolonged from two to 36 months after the end of the treatment. Eletrocardiographic drug-related changes were observed in seven out of eight patients (87,5%) without associated cardiologic disease and were not accompanied by cardiologic clinical manifestations. Eletroencephalographic drug-related changes were observed in three from nine patients in which the examination was performed. Poor therapeutic results in the vatients with ulcerative mucosal lesions, were observed as the drug experimented did not heale the mucosal lesions and only healed the cutaneous lesions in two of three patients who completed five treatment courses. Temporary and incomplete healing of the mucosal lesions was followed by relapse of them, after the end of therapy in all the patients with mucosal lesions. The author concluded that niridazole is not an effective drug in American cutaneous leishmaniasis with advanced metastatic lesions in mucous membranes of nose, mouth and/or pharynx. The author recommends vigilant observation of the patients during treatment due to possible appearance of neuro-psyehical symptoms.