The effect of heavy, habitual marijuana use compared with tobacco smoking on the composition of bronchoalveolar and peripheral blood lymphocytic phenotypes was examined. Bronchoalveolar lavage (BAL) and peripheral blood (PB) samples were taken from 14 nonsmokers (NS), 14 tobacco smokers (TS), 19 heavy, habitual marijuana smokers (MS), and 9 marijuana and tobacco smokers (MTS). In BAL fluid, marijuana use was associated with significantly higher alveolar macrophage concentrations, whereas tobacco smoking was associated with significantly higher alveolar macrophage, as well as higher bronchoalveolar lymphocyte and neutrophil concentrations. The bronchoalveolar T-lymphocytic phenotypic profiles of marijuana users differed from those of tobacco smokers. Tobacco, not marijuana, was found to have a significant effect toward lower percentages of bronchoalveolar CD4 cells, toward higher concentrations of bronchoalveolar CD8 cells, and toward lower bronchoalveolar CD4:CD8 ratios. Marijuana use had a significant effect toward lower percentages of bronchoalveolar CD8 cells. In peripheral blood, marijuana, but not tobacco, use was associated with significantly higher percentages of CD4 cells, lower percentages of CD8 cells, and higher CD4:CD8 ratios. These findings suggest that tobacco and marijuana have effects on bronchoalveolar and peripheral blood immunoregulatory T-lymphocytic subpopulations that differ in type or magnitude.