OBJECTIVE: Peripheral nerve ultrasound is an emerging tool in the diagnosis of carpal tunnel syndrome (CTS). Although numerous publications have cited an increased median nerve area at the wrist to be the diagnostic of CTS, there has been considerable variability in the published normal values for this measurement. Our objective is to collect data on the wrist-to-forearm ratio (WFR) of median nerve area in patients with CTS and healthy controls. METHODS: Patients with electrodiagnostically proven CTS underwent ultrasonography of the median nerve at the wrist and forearm. The median nerve area was measured at these points and compared to values from asymptomatic volunteers. RESULTS: The WFR of median nerve area in asymptomatic volunteers was 1.0+/-0.1. The WFR in patients presenting with CTS was 2.1+/-0.5. CONCLUSIONS: The WFR in patients with CTS is elevated as compared to asymptomatic controls. A WFR of 1.4 gave 100% sensitivity for detecting patients with CTS while using only median nerve area at the wrist resulted in a sensitivity of 45-93%, depending on the cut-off value used. SIGNIFICANCE: The WFR of median nerve area promises to be a valid means of diagnosing CTS, and may be superior to measuring median nerve area at the wrist alone.