Using two-color flow cytometric analysis, we studied peripheral blood lymphocyte subsets in 15 patients with human T-cell lymphotropic virus type I-associated myelopathy. The percentage of CD4+ 4B4+ cells (helper inducer T cell) was significantly increased in the patients with the myelopathy, compared with 16 healthy control subjects who were seronegative for human T-cell lymphotropic virus type I. However, there was no difference in the percentage of CD4+ 2H4+ cells (suppressor inducer T cell) between the two groups. The ratio of CD4+ 4B4+ cells to CD4+ cells and CD4+ 4B4+ to CD4+ 2H4+ cells was also elevated in these patients. The percentage of CD4+ DR+ cells and CD8+ DR+ cells, both of which are phenotypically activated T cells, and the ratio of CD4+ DR+ cells to CD4+ cells and of CD8+ DR+ cells to CD8+ cells are also increased in the patients, compared with the control subjects. The percentage of CD4+ 4B4+ cells showed positive correlations with values of spontaneous proliferation of peripheral blood lymphocytes and serum IgG level in patients with the myelopathy.