Case definitions and their relevancy to public health were discussed. The results of a recent case surveillance study of carpal tunnel syndrome (CTS) were summarized to illustrate the importance of defining criteria for case reporting and the differences between surveillance and clinical case definitions. Of 78 patients with upper extremity pain or paresthesia who were diagnosed with CTS on the basis of changes in median nerve conduction latency, only 68% were correctly diagnosed using symptoms, signs, and work histories. These clinical case criteria, when used separately, generally had high sensitivity but low specificity. The components of a medical surveillance program were discussed. These included detecting and reporting cases, analyzing and synthesizing the reports that have been received, and providing an appropriate response to the reports. Defining the cases carefully can facilitate each of the components. It was noted that for occupational illness surveillance, the critical issue is developing a case definition that will assist in identifying workplaces that require investigation and remediation. Any case definition used should have high sensitivity and should be easily applied to cases reported by primary care clinicians and specialists. The authors conclude that creating and applying appropriate surveillance case definitions can help bridge the gap existing between occupational and infectious disease surveillance.