Background. In 2003, human monkeypox was first identified in the United States. The outbreak was associated with exposure to infected prairie dogs, but the potential for person-to-person transmission was a concern. This study examines health care worker (HCW) exposure to 3 patients with confirmed monkeypox. Methods. Exposed HCWs, defined as HCWs who entered a 2-m radius surrounding case patients with confirmed monkeypox, were identified by infection-control practitioners. A self-administered questionnaire and analysis of paired serum specimens determined exposure status, immune response, and postexposure signs and symptoms of monkeypox. Results. Of 81 exposed HCWs, 57 (70%) participated in the study. Among 57 participants, 40 (70%) had ⩾1 unprotected exposure; none reported signs or symptoms consistent with monkeypox illness. One exposed HCW (2%), who had been vaccinated for smallpox within the past year, had serological evidence of recent orthopoxvirus infection; acute- and convalescent-phase serum specimens tested positive for anti-orthopoxvirus IgM. No exposed HCWs had signs and symptoms consistent with monkeypox. Conclusion. More than three-quarters of exposed HCWs reported at least 1 unprotected encounter with a patient who had monkeypox. One asymptomatic HCW showed laboratory evidence of recent orthopoxvirus infection, which was possibly attributable to either recent infection or smallpox vaccination. Transmission of monkeypox likely is a rare event in the health care setting.