PURPOSE: To prospectively evaluate the accuracy of sonography for diagnosis of cubital tunnel syndrome (CUTS) confirmed by electrodiagnostic study. ATERIALS and METHODS: From February 2004 to March 2005, we prospectively analyzed 24 elbows in 19 patients (8 women, 11 men; mean age, 49.2 years; range 23-65 years) with cubital tunnel syndrome, including 5 bilateral cases. Diagnoses of CUTS were confirmed by both clinical symptom and electrodiagnostic study. Sonographic findings of 20 asymptomatic cases served as controls. In sonographic examination, the cross sectional area of the ulnar nerve was measured at the inlet (at the level of medial epicondyle) and outlet (at the level of convergence of flexor carpi ulnaris tendons) of the cubital tunnel. The shape and echogenicity of the ulnar nerve were also evaluated, as were possible causes of entrapment. The accuracy of using ultrasonographic cross sectional area to diagnose CUTS was evaluated with receiver operating characteristic (ROC) analysis. RESULTS: The cross sectional area of the ulnar nerve at the inlet and outlet levels in CUTS patients was increased much more than in the asymptomatic cases. Decreased echogenicity and distortion of normal parallel echotexture of the ulnar nerve were observed in all CUTS patients. At the inlet level, the area under the ROC curve (AUC) was 0.816, and the ideal cut-off value for CUTS diagnosis was 0.08 cm2 with a sensitivity of 58.3%, specificity of 100%, positive predictive value (PPV) of 100%, and negative predictive value (NPV) of 66.7%. At the distal outlet level, the AUC was 0.785, and the cut-off value was 0.06 cm2 with a sensitivity of 79.2%, specificity of 70%, PPV of 76%, and NPV of 73.7%. When the summation value of inlet and outlet cross sectional areas was used, the AUC was 0.853, and cut-off value was 0.14 cm2 with a sensitivity of 70.8%, specificity of 85%, PPV of 85%, and NPV of 70.8%. CONCLUSION: Measurement of the cross sectional area of the ulnar nerve at the inlet and outlet of the cubital tunnel can aid in the sonographic diagnosis of cubital tunnel syndrome by more objectively determining the increase in cross sectional area.