Using an ultrasonic probe inserted into the mediastinum during cervical mediastinoscopy, mediastinal ultrasonography (USM) was performed on 63 patients with lung cancer. The patients with a small peripheral mass of less than 2 cm in diameter, according to the chest X-ray results, and with mediastinal lymph nodes smaller than 1 cm in their short axes as determined by computed tomography (CT), were excluded from this study. An analysis of the areas under the receiver operating characteristic curves derived from CT and USM showed that USM was superior (P = 0.043) to CT in terms of the diagnosis for mediastinal lymph node metastases, when the short axis dimension of mediastinal lymph nodes was employed for the diagnosis of metastases. The reason for this is that 97% of the mediastinal lymph nodes imaged by USM were located vertically along the body axis of the patient, and hence USM imaged the true short axis of the node in many cases. Our results indicate that USM is useful for performing a safe biopsy of lymph nodes during mediastinoscopy as well as for obtaining a clear imaging of the subcarinal nodes, which are inaccessible by normal cervical mediastinoscopy.