The diagnosis of carpal tunnel syndrome (CTS) is mainly based on clinical findings and electrodiagnostic tests (EDT). However, EDT results do not support clinical findings in some cases. It has been recently suggested that ultrasonography (US) can be used to diagnose CTS. In this study, we aimed to investigate whether US has a diagnostic value for CTS in patients with negative EDT findings or not. EDT was performed on 319 wrists with clinical CTS findings in electrophysiology laboratory. Median and ulnar nerve conduction velocities were measured in all cases and electromyography was performed in patient with tenar atrophy and having suspicion involvement of brachial plexus as EDT. Fifty-nine wrists with negative EDT (study group) and 30 wrists from 15 healthy individuals (control group) were examined using US. The mean of cross-sectional areas (CSAs) measurements were found 8.83+/-3.05 mm2 by tracing method (TM) and 8.51+/-3.13 mm2 by ellipsoid formula (EF) in study group, and 7.63+/-1.52 mm2 by TM and 7.66+/-1.42 mm2 by EF in control group. The differences between study group and control group according to both TM and EF were significant (t-test p=0.0079, p=0.0460, respectively). In study group, CSAs were larger than 10.5 mm2 in 18 (30.51%) and 16 (27.12%) wrists according to TM and EF findings, respectively, and in only one wrist (3.33%) in control group by both TM and EF. The differences of ultrasonographic CTS numbers between study group and control group were significant (p=0.0024 by TM, p=0.0086 by EF). We confirmed the usefulness of quantitative US assessment in the diagnosis of CTS in the patients with negative EDT findings. If EDT findings are inadequate to confirm the CTS in the patients with clinical CTS, US studies may be helpful to diagnose.