Summary We have characterized the T-lymphocytes in the skin lesions of 10 patients with cutaneous leishmaniasis and in the nasal lesions of seven patients with mucosal leishmaniasis. with the immunoperoxidase and monoclonal antibody techniques. There was predominance of cells with helper phenotype (Leu 3A + 3B) over suppressor phenotype (Leu 2a) in the lesions of both groups. The helper/suppressor (H/S) ratio in the skin lesions was 1±6 ± 0±5 and in the nasal lesions of mucosal leishmaniasis was 1±7 ± 0±8. The H/S ratios in the peripheral blood of patients with cutaneous leishmaniasis (2±1 ± 0±8) and in patients with mucosal leishmaniasis (1±6 ± 0±8) were comparable and similar to the ratios in the skin and nasal biopsies. The percentage of T-cells and macrophages expressing the Dr antigen in the cutaneous group (69±5 ± 13±7) was not significantly different from the mucosal patients (90±3 ± 5±7). We conclude that the immunopathology of the skin lesions in cutaneous leishmaniasis is similar to the nasal lesions of mucosal leishmaniasis.