Introduction: The aim of this study was to investigate factors affecting median nerve cross-sectional area (CSA) and determine a dependable swelling ratio (SR) calculation method. Methods: Using ultrasonography, median nerve CSAs were measured at the tunnel inlet, midtunnel, and outlet, and at 4 cm and 12 cm proximal to the wrist. We used CSAmax as the largest of the tunnel measurements and calculated swelling ratios (SRs) by using the CSAmax. Sonographic measurements were correlated with electrophysiologic findings. We evaluated the effects of gender, weight, and height on median nerve thickness. Results: We studied 95 wrists of 55 CTS patients and 48 wrists of 27 volunteers. Gender, weight, and height had effects on certain median nerve CSA measurements. CSAmax, SRmax4, and SRmax12 had superior correlations with electrophysiologic findings. The correlation between 4-cm and 12-cm median nerve CSAs was statistically significant (r = 0.879 and P < 0,001). Conclusion: We have developed a reliable SR method considering factors affecting median nerve CSA. © 2012 Wiley Periodicals, Inc., a Wiley company.