Immunoregulatory disorders have been implicated in the pathogenesis of asbestosis. We therefore compared the immunologic status of a well-characterized group of 31 current and former asbestos-cement workers with that of a group of 52 healthy controls, after adjustments had been made for the possible confounding effects of age, race, and smoking. The asbestos workers had significantly decreased percentages and numbers of both B and T lymphocytes in peripheral blood and a paradoxical IgG hypergammaglobulinemia. Analysis of T-lymphocyte subpopulations revealed that total T-cell numbers (OKT3+), helper-inducer T-cell numbers (OKT4+), and suppressor-cytotoxic T cell numbers (OKT8+) were decreased by similar proportions. These decreases were negatively correlated with time elapsing since the end of exposure to asbestos. In both workers and controls, lymphocyte proliferative responses to phytohemagglutinin (PHA) were correlated positively with the number of OKT4+ cells and negatively with age and serum IgG levels. When adjustments had been made for these confounding variables, no differences in PHA responses were noted between workers and controls. No relationship was detected in the workers between any of the immunologic aberrations noted and (1) radiographic category of pneumoconiosis, (2) estimates of cumulative asbestos exposure, or (3) abnormalities of pulmonary function. These data suggest that the immunologic perturbations we have noted in asbestos-exposed individuals are epiphenomena, unrelated to the pathogenesis of asbestosis itself.