The National Institute for Occupational Safety and Health (NIOSH) has proposed a surveillance case definition for work-related carpal tunnel syndrome (CTS). The case definition requires the presence of median nerve symptoms; one or more occupational risk factors; and objective evidence of CTS including one of three physical examination findings or nerve conduction tests diagnostic of CTS. We evaluated the performance of the NIOSH case definition, restricting our analysis to cases in which physical examination findings served as the objective criterion. Nerve conduction studies were used as the gold standard. Seventy-eight workers were studied; 38 percent had CTS. The NIOSH case definition had sensitivity of 0.67 (95% CI = 0.57, 0.77), specificity of 0.58 (95% CI = 0.47, 0.69), and positive and negative predictive values of 0.50 (95% CI = 0.39, 0.61) and 0.74 (95% CI = 0.64, 0.84), respectively. Overall 38 percent of subjects were classified incorrectly. In a sample with a prevalence of 15 percent, as might be encountered in high risk workplaces, the positive predictive value would be 0.22. In conclusion, when physical examination findings serve as the objective criterion the performance of the case definition is modest reflecting the limited diagnostic value of its component tests and indicating that effective screening for CTS awaits improved diagnostic techniques.