We investigated whether serologic evidence of lymphadenopathy-associated virus (LAV), an exogenous human T-cell lymphotropic and cytopathic retrovirus, correlated with the acquisition and transmission of the acquired immunodeficiency syndrome (AIDS). Serum from 17 of 25 patients with AIDS contained circulating IgG anti-LAV antibody (all of 5 adults with cancer and 6 of 12 adults and 6 of 8 children with opportunistic infections, with or without Kaposi's sarcoma). All of eight homosexual men with generalized lymphadenopathy or the AIDS-related complex and five homosexual men with AIDS prodromes who subsequently had AIDS were also seropositive. The anti-LAV antibody was not found in 99 of 100 healthy blood donors or in 23 patients with genetic immunodeficiencies, supporting the contention that LAV is unlikely to represent simply another opportunistic microorganism. An AIDS carrier state was indicated by the probable transfer of LAV from asymptomatic, immunologically competent mothers to their offspring. These data offer a basis for the hypotheses that LAV is a marker for AIDS, may be carried and transmitted in the absence of clinical findings or in vitro T-cell abnormalities, and is probably an etiologically important agent in AIDS and its prodromes.