For selected diagnoses of public health interest during the 1996 Olympic Games, the authors compared data concurrently obtained on the same patient population by two separate surveillance systems: (1) an existing hospital electronic medical billing records system and (2) a system based on manual record abstraction. Counts of total patient visits closely agreed, though the two systems differed considerably in some diagnostic categories, especially injuries. The authors concluded that while causation, risk factors, and illness severity are not reflected directly in standard International Classification of Diseases (ICD) codes, and "E" codes to indicate causation may not be used, special-purpose surveillance systems based on existing computerized medical records may be as effective as manual data abstracting.