In view of the strong association between the acquired immunodeficiency syndrome (AIDS) and sexually transmitted diseases (STDs), we screened 182 human immunodeficiency virus (HIV)-1 infected patients over a 15-month period for serological markers to previously encountered or current STDs, most of viral etiology. The relationship between their immunological and clinical status and the prevalence of STDs was assessed and compared with that of 88 HIV-seronegative patients. Hepatitis B virus and Treponema pallidum were the most frequently occurring pathogens in both HIV-1-infected and HIV-seronegative patients. Hepatitis C virus (HCV) infection was also observed in both groups, but no HIV-seronegative patient was infected with human T-lymphotropic virus type 1 (HTLV-1). The Centers for Disease Control clinical staging of A1 through C3, representing asymptomatic to severe AIDS conditions, was observed in HIV-1 patients with or without STDs. A mean CD4 count of 288 cells per microliter (95% CI of 237-340 cells per microliter) in HIV-1 patients was significantly lower (P < 0.05) than that in HIV-seronegative individuals with 1019 cells per microliter (95% CI of 924-1115 cells per microliter), irrespective of whether subjects in either group had previous or current STDs. The mean CD4 count of patients with a single infection from HIV-1 was not significantly different (P = 0.36) from that of HIV-1 patients with multiple infections. HIV-1 infection alone appears to be responsible for the marked immunodeficiency status of seropositive patients observed in this study.