To characterize health care workers with the acquired immunodeficiency syndrome (AIDS) in the United States and to evaluate the role of occupational transmission of the human immunodeficiency virus (HIV).National AIDS surveillance data.Health care workers with AIDS are reported to the Centers for Disease Control by state and local health departments. Health care workers who do not report a nonoccupational risk for HIV infection are termed undetermined risk cases and are investigated by health departments using a standard protocol.Through June 30, 1990, there were 5425 cases of AIDS in health care workers reported in the United States. Three of these workers developed AIDS following well-documented occupational exposure to HIV-infected blood. Of the 539 health care workers initially reported without a nonoccupational risk, follow-up investigations were completed for 303. Nonoccupational risk factors were established for 237 (78.2%) of the 303 investigated health care workers; 66 workers (21.8%) remained in the undetermined category. Follow-up information was incomplete for 236 health care workers who also remained in the undetermined category, resulting in 5120 health care workers (94.4%) with AIDS with nonoccupational risks for HIV infection. Overall, health care workers were more likely than non-health care workers with AIDS to have an undetermined risk for HIV infection (5.6% vs 2.8%; P less than .001). While many of the 66 investigated health care workers had jobs involving contact with patients and/or potential contact with blood, none reported percutaneous, mucous membrane, or cutaneous exposures to blood or body fluids known to be infected with HIV.Surveillance data suggest that most health care workers with AIDS acquired their HIV infection through a nonoccupational route.