Despite the postulated role of the immune system in the control of the infection by Paracoccidioides brasiliensis, only a few studies have addressed this point in patients. The determination of total lymphocytes and their subpopulations in 6 untreated patients with the chronic form of paracoccidiodomycosis showed that half of them were lymphopenic, because of low number of CD4+ T-lymphocytes. All patients had low CD4/CD8 ratios. On the contrary, B-lymphocytes were normal in all patients. An additional patient, studied on treatment with ketoconazole, had normal lymphocyte counts in all subpopulations, as did one of the patients previously studied at diagnosis when he received specific antimycotic treatment. The production of interferon and tumor necrosis factor, determined by bioassay in supernatants of mononuclear blood cells of the patients, induced by interleukin 2 in vitro was significantly lower than that of normal subjects. These results show that patients with paracoccidioidomycosis have a defect in blood lymphocyte subsets as well as in the ability to produce regulatory cytokines.