Human endogenous retrovirus (HERV)-like sequences are normal inherited elements that constitute several hundredths of the human genome. The expression of genes located within these elements can occur as a consequence of several different events, including persistent inflammation or genotoxic events. Antibodies to endogenous retroviral gene products have been found in a number of infectious, chronic, and malignant diseases, suggesting a role in disease initiation and progression. We studied human immunodeficiency virus type 1 (HIV-1)-infected patients for evidence of urine antibody to a HERV peptide and investigated correlates with clinical and laboratory parameters. Forty-three HIV-1-infected patients in documented asymptomatic, symptomatic, or AIDS stages of disease and 21 age- and gender-matched, uninfected controls were tested for antibody to HERV-related peptide 4.1. Urine specimens were examined in a blinded fashion with the Calypte Biomedical Corp. experimental enzyme immunoassay for antibody to peptide 4.1. Results were compared with demographic data, medical history, clinical state of disease, and results of other laboratory tests. Thirty-six percent of the asymptomatic (Centers for Disease Control and Prevention [CDC] category A) and 81.3% of both the symptomatic (CDC category B) and AIDS (CDC category C) patients were positive for antibody to HERV-related peptide 4.1. None of the controls were positive. In this study, antibodies to HERV-related peptide 4.1 were found more frequently in patients with advanced stages (categories B and C) of HIV-1 disease than in those patients with an earlier stage (category A) of HIV disease. In HIV patients, severe immunosuppression, defined as having had at least one opportunistic infection, correlated with the expression of antibody to a HERV-related peptide.