The immune systems of patients with AIDS are characterized by a profound defect in cell-mediated immunity which is predominantly due to a decrease in the number and function of the helper/inducer T lymphocytes, particularly the antigen-reactive cells. This defect is manifested primarily as decreases in delayed-type hypersensitivity reactions and decreased in vitro proliferation to soluble antigen. A variety of secondary manifestations of immunologic dysfunction occur, some of which result from a lack of effective inducer-cell function, others from the occurrence of opportunistic infections. Among these secondary phenomena are decreased cytotoxic lymphocyte responses, polyclonal B-cell activation, decreased monocyte chemotaxis, and a number of serologic abnormalities. The primary cause of this defect in the antigen-reactive helper/inducer T lymphocyte is infection with a class of T-cell tropic retroviruses known as HTLV-III or LAV. This virus is capable of selectively infecting T4 + lymphocytes and can be isolated consistently from patients with AIDS or AIDS-related conditions. Despite substantial knowledge concerning the nature of the immune defect in AIDS and its causative agent, little progress has been made in developing effective therapies for this uniformly fatal illness. Because the incidence of this disease continues to increase, and patients stricken with this illness have a median survival of two years, additional investigation in this area is greatly needed. Continued effort aimed at delineating the precise nature of the immune defect in these patients should be of value in attempting to enhance our understanding of the human immune system in both normal and disease states.