Seroprevalence of human T lymphotropic virus (HTLV) and human immunodeficiency virus type 1 (HIV-1) was determined among 7841 intravenous drug users (IVDUs) from drug treatment centers in Baltimore, Chicago, Los Angeles, New Jersey (Asbury Park and Trenton), New York City (Brooklyn and Harlem), Philadelphia, and San Antonio, Texas; 20.9% had evidence of HTLV infection, as determined using a p21e EIA for screening and p21e blot for confirmation. With a type-specific EIA and blot used in combination, HTLV-II was identified in 97.6% of HTLV-positive IVDUs whose sera could be subtyped. HIV-1 seroprevalence was 13.2%. HTLV-II without HIV-1 was most common in Los Angeles and San Antonio. HIV-1 without HTLV-II was most common in New York, New Jersey, and Baltimore. Dual infection was most common in New York and New Jersey. Logistic regression analysis revealed that seroprevalence of HTLV-II was significantly greater with HIV-1 infection and increasing age and among women, blacks, and Mexican-Americans. In conclusion, it appears that among US IVDUs, nearly all HTLV infection is attributable to HTLV-II, and HTLV-II infection is associated with HIV-1 and sociodemographic background.