Persistent generalized lymphadenopathy (PGL) is observed predominantly in subjects at risk of developing AIDS. Twenty-seven individuals belonging to such groups: twelve homosexual males and fifteen intravenous drug users, were investigated for immunological abnormalities with particular attention to monocyte functions. They were compared with five AIDS patients. Twenty out of twenty-two individuals had anti-LAV/HTLV-III antibodies and most had abnormalities characteristic of AIDS: polyclonal hypergammaglobulinemia, decreased cell-mediated immunity, inverted T-cell helper/suppressor ratio and histological alterations of lymph nodes. As for peripheral blood monocyte functions, phagocytic capacity and production of O2- were normal and bactericidal capacity was decreased. Monocytes cultured in the presence of concanavalin A produced less PGE2 and more IL-1/MCF than normal monocytes. Similar abnormalities were found using monocytes from AIDS patients. These data suggest that monocytes from patients with PGL have functional alterations that may be either intrinsic or secondary to lymphocyte dysfunction(s); these alterations do not account for the decreased capacity of lymphocytes to respond to mitogens but may explain the uncontrolled activation of B cells.