The relationship between smoking and percentages and counts of T-cell subsets, in particular CD45R0+ (memory) and CD45RA+ (naive) CD4+ lymphocytes, have been assessed in a sample of 311 middle-aged men working in the Paris area. Percentages of lymphocytes with the phenotypes CD4+, CD8+, CD4+CD45R0+, and CD4+CD45RA+ were determined using double immunofluorescence labeling. All cell counts, including CD4+CD45R0+, and CD4+CD45RA+ lymphocytes increased significantly with tobacco consumption, as did the percentages of lymphocytes with the CD4+CD45R0+ and CD4+ phenotypes. The increase in the percentage of lymphocytes with the CD4+CD45RA+ phenotype, although not significant, was similar to that observed in the percentage of CD4+CD45R0+ lymphocytes. When the proportions of CD4+ cells with the CD45R0+ or the CD45RA+ phenotype were considered, no specific modification of any of these two sub-populations was observed: the effect of smoking on memory and naive Cd4+ lymphocytes seemed to be equivalent.