A human retrovirus was isolated from the peripheral blood of three American patients newly diagnosed with the acquired immunodeficiency syndrome (AIDS). In each case the major core viral protein (p25) was shown to be antigenically identical to that of the prototype lymphadenopathyassociated virus (LAV). Two of the viral isolates were derived from intravenous narcotics abusers, the first demonstration of LAV isolation from this risk group. Antibody to LAV was detected by an IgG enzyme-linked immunosorbent assay in the serum samples of these and 14 additional American patients with AIDS and in none of 12 hospital worker controls. These findings provide support for the etiologic association of LAV and AIDS.