OBJECTIVE This study defined the incidence rate of carpal tunnel syndrome (CTS) among active assembly workers and evaluated risk factors. METHODS This study followed 189 automobile assembly workers over 1 year. Incident cases were defined as workers who had no current or prior history of CTS and were subsequently diagnosed with CTS by means of active surveillance during the study. RESULTS The estimated annual incidence rate ranged from 1% to 10%, depending on the case definition. Significant predictors for CTS include a higher baseline median-ulnar peak latency difference, history of wrist/hand/finger tendonitis, history of diabetes, nonneutral wrist and elbow postures, and a lower self-reported social support. CONCLUSIONS This is similar to other prospective studies that demonstrate both ergonomic and medical history are independent risk factors for development of CTS.