An association of HTLV-I virus with myelomonocytic leukemia was established by serologic and molecular-biologic procedures. Antibodies to one or two HTLV-I proteins were identified in 10 out of 30 patients with myelomonocytic leukemia; both antibodies to certain HTLV-I proteins and an integrated provirus were detected in 3 cases. Upon examination of donor blood, antibodies to one or two HTLV-I proteins were found in 0.8% cases, but only 0.3% of the examined donors were found positive to HTLV-I. This finding points to the presence of said virus in the population of Latvia.