Seventy untreated paracoccidioidomycosis patients, 15 with the acute or subacute form of the disease and 55 with the chronic form, were compared with two normal control groups of the same age range. Peripheral blood mononuclear cell subsets were defined by monoclonal antibodies directed at total T cells, helper/inducer and suppressor/cytotoxic T cell subpopulations; B cells, cortical thymocytes and monocyte/null cells. Both groups of patients showed an increased number of monocyte/null cells, a low helper/suppressor ratio and a reduced percentage of total T cells and their helper/inducer subsets. In addition patients with the acute form of the disease exhibited high levels of suppressor/cytotoxic T cells and B cells. These findings are of importance in our attempts to understand the pathogenesis of this mycosis and also to evaluate its prognosis in individual patients.