The risk of nosocomial human immunodeficiency virus (HIV) infection among hospitalized patients comes almost exclusively from transfusion of fresh blood products. Current estimates of the risk of HIV infection from the transfusion of blood or components vary from 1/40,000 to 1/250,000 (0.0025% to 0.0004%), with the most probable likelihood estimated to be 1/153,000 (0.0007%). The major route of transmitting such HIV infection is via blood collected during the interval between infection of the donor and development of a detectable circulating antibody level to the AIDS virus (i.e., the "window period"). The current risk to hemophiliacs receiving treated coagulation factor concentrates is negligible. The risk to healthcare personnel of acquiring HIV infection from accidental puncture wounds and from handling HIV-infected blood or body fluids is 0.42% per episode. Most reported seroconversions have resulted from penetrating injuries with sharp objects contaminated with HIV-positive blood. The degree of risk to healthcare workers will vary with the community, the patient population served, and the frequency of penetrating injuries.