The value of ultrasonic examination and ultrasonically guided needle biopsy (UGNB) of mediastinal masses was investigated in 45 patients. Fifteen of the masses were malignant and 30 were benign. The diagnoses were histologically confirmed by examination of surgical or autopsy specimens. Only teratoma could be confidently diagnosed by ultrasonography alone. Diagnosis of other types of mediastinal masses required examination of tissue samples. Using ultrasonically guided aspiration biopsy (UGAB), we were able to diagnose 13 of 15 malignant tumors and 18 of 27 benign mediastinal masses. Ultrasonically guided cutting biopsy was performed in conjunction with UGAB in 11 patients, and the diagnosis was histologically confirmed in 9 cases. Complications with UGNB occurred in only 1 case. Our results indicate that UGNB is a safe, simple technique that can be applied to most of the mediastinal masses. We believe that it is the method of choice in such cases.