A case of diffuse cutaneous leishmaniasis (DCL) acquired in Peru is described. The causative agent was Leishmania mexicana amazonensis as determined by isoenzyme analysis and species-specific monoclonal antibody binding characteristics. Histological examination of biopsy material showed a large number of intracellular and extracellular amastigotes and few lymphocytes. Treatment with meglumine antimoniate (Glucantime) administered iv at a dosage of 20 mg antimony/kg body weight/day for 60 days resulted in visible improvement of the lesions, but not in clinical or parasitological cure.