The expression of CD3, CD4 and CD8 antigens was simultaneously evaluated in peripheral blood and bone marrow lymphocytes from 22 multiple myeloma (MM) patients. The coexpression of CD11b and HLA-DR antigens was also analyzed within the CD4+ and CD8+ subpopulations in 4 MM patients. In peripheral blood the percentage of CD3+ and CD8+ cells was in the normal range, and the percentage of CD4+ was slightly reduced, leading to an altered CD4/CD8 ratio (less than 1) in only 7 patients. On the contrary, in bone marrow the percentage of CD4+ was profoundly reduced, leading to an altered CD4/CD8 ratio in all MM patients. As we previously reported in peripheral blood, an expansion of the CD11b+ and HLA-DR+ lymphocytes was observed within the CD4+ and CD8+ bone marrow T-cell subpopulations. A T-lymphocyte subset imbalance is more evident in bone marrow than in peripheral blood, and it is not due to a different lymphocyte distribution within the hematological compartments.