A limiting dilution analysis (LDA) was utilized to estimate the frequency of L. braziliensis braziliensis reactive T cells (Lbb-T cells) in peripheral blood and in the lesions of patients with mild localized cutaneous leishmaniasis (LCL) or with severe mucosal leishmaniasis (MCL). The frequencies of Lbb-T cells in peripheral blood varied from 1:107300 to 1:3587 and were not significantly different in MCL and LCL patients. However, a significant difference was encountered (P less than 0.02) between the T cells frequencies in cutaneous (1:748 to 1:45) and mucosal lesions (1:152 to 1:13). A positive correlation was also observed between these frequencies and the magnitude of delayed-type hypersensitivity (DTH) (P less than 0.01) and the presence of fibrinoid necrosis and granulomatous reaction in the site of the lesions (P less than 0.05). The lack of correlation between the severity of disease (MCL or LCL) and the frequency of Lbb-T cells in peripheral blood gave no indications towards understanding the physiopathology of severe or mild disease. However, the correlation between high T cell frequencies in the site of the lesions, the magnitude of DTH, the fibrinoid necrosis and the severity of the disease (MCL lesions) points to the possibility that the presence of a strong T cell dependent cellular immune response in the site of the lesions may have a deleterious effect. However, a local well modulated T cell immune response might provide healing of the lesions.