T-cell subsets with a naive phenotype are selectively decreased in the peripheral blood of patients with mycosis fungoides. Peripheral blood lymphocytes of 33 patients with histopathologic confirmation of mycosis fungoides and 27 healthy controls matched for age and sex were analyzed with a panel of monoclonal antibodies using both single and dual color immunofluorescence. Patients with mycosis fungoides had a significant reduction in the percentage of circulating T cells with a naive phenotype (i.e., CD4+2H4+ and CD4+Leu8+), as well as a significant reduction in the absolute numbers of circulating lymphocytes with the phenotype CD8+Leu8+ compared to the control cohort. The reduction in circulating naive T cells was found to occur irrespective of stage of disease, duration of disease, or mode of treatment. The depletion of circulating naive T cells may reflect increased conversion to memory T cells in the peripheral blood or skin. 