An end-viewing fiberoptic gastroscope was modified to incorporate a high-resolution linear array real-time ultrasound transducer 7 cm proximal to the flexible tip of the instrument. This endoscope was used to evaluate 20 patients suspected of having mural disease of the upper gastrointestinal tract and five normal subjects. The endosonographic examination defined esophageal, stomach, and duodenal wall thickness in normal subjects. In patients with pathological findings, both diffuse and acoustically focal lesions were demonstrated, and the extent of these lesions were outlined. With focal masses, endosonographic findings also demonstrated echogenic differences between various submucosal tumors reflecting their differing etiology. The extent of invasion and adenopathy were identified in a patient with gastric carcinoma. With diffuse lesions, the depth of involvement could be outlined sonographically, but etiologic diagnosis depended on biopsy. Extrinsic masses with secondary invasion of the wall of the upper gastrointestinal tract were defined in six patients. It is suggested that endosonography may prove to be a useful technique for evaluation of mural, invasive extramural, and diffuse lesions of the upper gastrointestinal tract.