Eleven patients with Pancoast tumor, who failed to yield diagnostic materials by conventional sputum cytology and fiberoptic bronchoscopy, were studied by real-time linear-array and sector ultrasonography. The sector scanner through the supraclavicular approach adequately visualized the external profile and the internal texture of the lesions in all 11 patients, which is a significant improvement (p less than 0.05) over what can be accomplished with linear-array scanner through the intercostal approach. All patients received percutaneous transthoracic aspiration under ultrasound guidance. Positive cytologic diagnosis was established in ten of the 11 patients (91 percent). Additional biopsies performed in seven patients under similar ultrasonic guidance also provided concordant results. No complications were observed in this series. This study has clearly shown that ultrasound-guided aspiration biopsy can be a safe and useful means for obtaining materials for pathologic confirmation of Pancoast tumor. It may also assist in defining the tumor extension to pleura and adjacent structures.