We characterized T-lymphocyte subsets in peripheral blood of smokers (N = 60) and nonsmokers (N = 35). Total T-lymphocytes and T cell subsets were similar to nonsmokers in light and moderate smokers. In heavy smokers, total OKT3+ cells were increased, the percentage of OKT4+ cells was decreased, and percentage and total number of OKT8+ cells were increased. The ratio of OKT4+ to OKT8+ lymphocytes was decreased in heavy smokers. The percentage of OKT8+ cells and the OKT4+/OKT8+ ratio returned to normal in heavy smokers six weeks after they stopped smoking. These findings suggest that cigarette smoking causes reversible alterations in immunoregulatory T cells.