Summary:We surveyed for serologie evidence of either HIV-1 or HTLV-I/II infection in 387 male veterans who entered into an inpatient drug treatment center. Serum was obtained after receiving written informed consent. Serum specimens were tested by enzyme-linked immunosorbent assay for antibody to HIV-1 and for antibody to HTLV-I/II; sera that were repeatedly reactive were then tested by Western blot (HIV-1/HTLV-I/II) and radioimmunoprecipitation assay (HTLV-I/II). Sixty-five of 387 (16.79%) patients were tested and confirmed as positive for HTLV-I/II only antibodies and 30 of the 387 (7.75%) were positive for HIV-1 only antibodies. An additional nine patients (2.32%) were seropositive for antibodies to both viruses. A statistically significant difference in the CD4/CD8 lymphocyte ratio was associated with HIV-1 sero-positivity. HTLV-I/II seropositivity was strongly associated with black race, age, and duration of i.v. drug use, but not with sexual intercourse as determined by lifetime history of number of sexual partners, incidence of sexually transmitted diseases, type of drug used, or needle-sharing practices.