Data from a continuing multiyear seroprevalence survey of human T-lymphotropic virus types I or II (HTLV-I/II) among intravenous drug users in seven U.S. locations were analyzed to detect demographic patterns of seropositivity and coinfection with human immunodeficiency virus type 1 (HIV-1). Seropositivity for HTLV-I/II and HIV-1 was detected by whole-virus enzyme immunoassay, with Western blot confirmation. Of 1,800 subjects recruited from methadone maintenance and detoxification clinics, 207 (11.5%) were infected with HTLV-I/II. Seropositivity for HTLV-I/II varied by racial group, age, sex, and geographic location. Blacks had a higher (age- and location-adjusted) infection rate (17.1%) than Hispanics (8.7%) or whites (5.6%), and seropositivity showed a strong gradient with increasing age. Females had a slightly higher rate (14.0%) than males (10.0%), after adjustment for age and location. Among the seven locations, the rate varied from approximately 1% (Miami and Baltimore) to 20% (Los Angeles), although the former rates were based on relatively few subjects (47 and 65, respectively). Overall, the occurrence of coinfection by HIV-1 and HTLV-I/II did not occur more frequently than expected by chance.