Patients with human T-cell leukemia virus type-1 (HTLV-1)-associated myelopathy (HAM/TSP) generally harbor a much greater population of HTLV-1-infected T cells in peripheral blood mononuclear cells (PBMCs) than asymptomatic carriers. These cells are not malignant but frequently proliferate clonally. To investigate the possibility that higher expression of the viral activator Tax induces preferential proliferation of infected nonmalignant T cells in HAM/TSP patients, the expression of Tax mRNA in fresh PBMCs was analyzed by reverse transcriptase-mediated polymerase chain reaction. Total amount of Tax mRNA was higher in HAM/TSP patients than in carriers, but the expression level was almost the same as that in asymptomatic carriers when compared per infected cell. The expression levels in adult T-cell leukemia patients were significantly lower than those in HAM/TSP patients and asymptomatic carriers. These results indicate that tax gene is not expressed at a continuously high level in HAM/TSP patients who carry a high population of infected T cells, even in those with clonally proliferated infected T cells.