Serum specimens from patients attending sexually transmitted disease (STD) clinics in Denver and several southeastern US cities were tested for antibody to human T-lymphotropic virus type I or II (HTLV-I/II). In Denver, 8 (2.1%) of 384 patients with a history of intravenous (IV) drug use, versus none of 201 non-IV-drug users, were HTLV-I/II seropositive. Only 2 (0.18%) of 1095 STD clinic patients from the southeastern USA had antibodies to HTLV-I/II. These data document a low prevalence of HTLV-I/II in STD clinic patients from the southeastern USA and confirm that IV drug use is an important risk factor for HTLV-I/II in the USA.