To calculate incidence and prevalence rates, each center identified the appropriate study population for analysis purposes in collaboration with the coordinating center. For prevalence calculations, the study population consisted of donors at all centers who indicated that they had never donated blood previously. To calculate incidence, the study population consisted of donors who had donated previously in the same center. The incidence of retroviral infections was assessed annually in years three through five. Clinical follow-up of all subjects concluded 48 months after the beginning of enrollment. A blood profile for each center was required to calculate denominator values for incidence and prevalence determinations. The blood profile, or donor demographic characterization consisted of information regarding total donor volume with respect to donation status; new or repeat donor, and if repeat, the time of last donation; age; sex; and race.