Objective. - Participation in competitive sports is popular and widely encouraged throughout the United States. Reports of infectious disease outbreaks among competitive athletes and recent publicity regarding infectious disease concerns in sports underscore the need to better characterize the occurrence of these problems. Data Sources. - To identify reports of infectious diseases in sports, we performed a comprehensive search of the medical literature (MEDLINE) and newspaper databases in two on-line services (NEXIS and DIALOG PAPERS). Study Selection. - Articles selected from the literature review included those describing cases or outbreaks of disease in which exposure to an infectious agent was likely to have occurred during training for competitive sports or during actual competition. Articles from the newspaper review included reports of outbreaks, exposures, or preventive measures that directly or indirectly involved teams or spectators. Data Synthesis. - The literature review identified 38 reports of infectious disease outbreaks or other instances of transmission through person-to- person (24 reports), common-source (nine reports), or airborne (five reports) routes; the newspaper search identified 28 reports. Infectious agents included predominantly viruses but also a variety of fungi and gram-positive and gram-negative bacteria. Conclusions. - Our findings indicate that strategies to prevent transmission of infectious diseases in sports must recognize risks at three levels: the individual athlete, the team, and spectators or others who may become exposed to infectious diseases as a result of sports-related activities. Team physicians and others who are responsible for the health of athletes should be especially familiar with the features of infectious diseases that occur in sports and measures for the prevention of these problems.