INTRODUCTION: Sonographically measured median nerve cross-sectional area (CSA) at the wrist is increased in patients with carpal tunnel syndrome (CTS). Ultrasound of the median nerve may be useful in screening for electrodiagnostic (EDx) abnormalities. METHODS: EDx studies were performed on all participants. Sonographic evaluation of median nerve CSA at the wrist and in the forearm was performed by a physician blinded to the EDx results. Multivariate and receiver operating characteristic (ROC) analysis were used to assess the data. RESULTS: Fifty control wrists and 192 symptomatic wrists were analyzed. Sensitivity of ultrasound in excluding EDx abnormality was 99% for CSA of 9 mm(2) and 97% for a wrist-forearm ratio (WFR) of 1.4. There was no clinically significant correlation between ultrasound parameters and EDx severity. CONCLUSIONS: Median nerve ultrasound is a highly sensitive screening tool for EDx abnormality, but it cannot determine severity. This may lead to changes in the current standards for evaluating CTS.