Human retroviruses, although recently identified, have been the focus of much attention since they are linked to neoplasia and chronic degenerative diseases. The human T-cell lymphotropic virus type I (HTLV-I), isolated in 1980, is the etiologic agent of human adult T-cell leukemia and HTLV-I associated myelopathy (HAM). In 1982, a second type of HTLV was described (HTLV-II); lately antibodies to HTLV-Il were found to be highly prevalent in IV-drug users.