Increased provision of health care in outpatient settings and concerns about occupational transmission of infections have focused attention on the risk of transmission of infectious diseases in ambulatory health care settings. In contrast to inpatient nosocomial infections, infections transmitted in outpatient settings are neither systematically monitored nor likely to be detected by routine qi surveillance. To better define the spectrum of such events, we reviewed the literature to identify cases and clusters of infections associated with outpatient health care. In this review, we identified and epidemiologically characterized 53 such reports that occurred from 1961 through 1990. Transmission occurred in general medical offices, clinics, and emergency departments (23); ophthalmologists' offices and clinics (11); dental offices (13); and alternative-care settings (six). Our findings suggest that inpatient infection-control practices should be extended to outpatient health care settings by assigning specific responsibility for infection control and by adapting surveillance methods and prevention measures.