Human T-cell leukemia (or T-lymphotropic) virus type I (HTLV-I) is a human exogenous infectious retrovirus of the family Retroviridae. This virus has been associated with adult T-cell leukemia and endemic myelopathies (tropical spastic paraparesis and HTLV-I associated myelopathy). HTLV-I is transmitted by sexual contact, from mother to child, by intravenous drug abuse, and by blood transfusion. The estimated lifetime risk of developing disease in antibody-positive patients is 1 in 80, and a latency period as long as 20 years can intervene. No case of transfusion-transmitted disease has been reported to date. Currently, no testing of blood donors for HTLV-I is required in the United States, and no such test has been approved by the Food and Drug Administration. Because data on the natural history of this virus may take years to accumulate, it is probably wise to begin excluding anti-HTLV-I-positive units from the blood supply in the United States as soon as a licensed test is available.