The purpose of this study was to develop a clear picture of what immunologic changes occurred in patients who were transfused and to determine when these changes occurred and how long they lasted. Since most of the patient population was heavily transfused, some of the study subjects eventually developed AIDS. Information obtained during the study about these patients was then used to answer such questions as: Which factors determined a particular person's risk of developing AIDS and to what degree? What alterations occurred before the clinical manifestations of AIDS? Was progression from normal to defective cell-mediated immunity to AIDS strictly unidirectional or did altered cell-mediated immunity in the study cohort ever regress to normality? If progression was not unidirectional, what determined the outcome? Also, the importance of establishing a serum and cell repository could not be overemphasized, since, as new scientific information became available, these specimens enabled the performance of a number of different retrospective studies.