Human T-lymphotropic viruses types I and II (HTLV-I and HTLV-II) cause chronic infections of T lymphocytes that may lead to leukemia and myelopathy. However, their long-term effects on blood counts and hematopoiesis are poorly understood. We followed 151 HTLV-I-seropositive, 387 HTLV-II-seropositive, and 799 HTLV-seronegative former blood donors from 5 U.S. blood centers for a median of 14.0 years. Complete blood counts were performed every 2 years. Multivariable repeated measures analyses were conducted to evaluate the independent effect of HTLV infection and potential confounders on 9 hematologic measurements. Participants with HTLV-II had significant (P < .05) increases in their adjusted lymphocyte counts (+126 cells/mm(3); approximately +7%), hemoglobin (+2 g/L [+0.2 g/dL]) and mean corpuscular volume (MCV; 1.0 fL) compared with seronegative participants. Participants with HTLV-I and HTLV-II had higher adjusted platelet counts (+16 544 and +21 657 cells/mm(3); P < .05) than seronegatives. Among all participants, time led to decreases in platelet count and lymphocyte counts, and to increases in MCV and monocytes. Sex, race, smoking, and alcohol consumption all had significant effects on blood counts. The HTLV-II effect on lymphocytes is novel and may be related to viral transactivation or immune response. HTLV-I and HTLV-II associations with higher platelet counts suggest viral effects on hematopoietic growth factors or cytokines.