To provide medical services at mass gatherings for scheduled special events (e.g., world fairs, music festivals, and athletic competitions such as the Olympics), organizers must have information to anticipate both routine and uncommon situations. In November 1993, approximately 9500 athletes and staff from 31 countries participated in the XVII Central American and Caribbean Games in San Juan, Puerto Rico. To monitor injury- and illness-related morbidity among participants, the schools of public health and medicine at the University of Puerto Rico and the Puerto Rico Olympic Committee established a public health surveillance system designed specifically for this event. This report summarizes selected results from the system, which underscore the usefulness of this approach in planning prevention, medical, and emergency services for similar events.