The degree of coinfections with blood-borne or sexually transmitted pathogens (HIV-1, HTLV-I/II, HBV, HCV, HDV, and Treponema pallidum) were assessed in individuals attending sexually transmitted diseases (STD) clinic and patients admitted to a hospital through the emergency room in Baltimore. Enzyme-linked immunosorbent assays (ELISA), immunoblots, and card tests were used to screen the sera. Nearly one third of the individuals in both populations were infected with one or more pathogens. With some minor exceptions, all individuals with dual or multiple infections had antibodies reactive with the HBV core antigen. There was a strong overall association between the presence of antibodies to HIV-1 and the presence of antibodies to HBV core and HCV in both populations. Additionally, the presence of HIV-1 antibodies was significantly associated with the presence of HTLV-I/II antibodies and HBV surface antigen in the STD population and with a positive RPR test result in the H/ER population. We suggest that HIV-1 and/or HTLV-I/II infected individuals in STD clinic and emergency rooms are highly likely to have had past infections with HBV or HCV.