The Albuquerque branch of the United Blood Services system was found to have an unusually high blood donor human T-cell leukemia/lymphoma virus (HTLV) seroprevalence (0.72 per 1,000). Many studies investigating HTLV seroprevalence and transmission have assumed that all seropositivity is due to HTLV type I (HTLV-I); recent data dispute this conclusion. We investigated the high prevalence of HTLV seropositivity in New Mexico by determining whether HTLV-I or HTLV-II is predominant in our donors. Using polymerase chain reaction (PCR) amplification of proviral DNA from peripheral blood, followed by sequence-specific hybridization with oligonucleotide probes to distinguish the two viruses, we demonstrate that 9 of 10 Western blot-confirmed HTLV-seropositive blood donors from New Mexico are infected with HTLV-II. Implications of this finding for donors and the safety of the blood supply are discussed.