Penetration of gastric cancer through the wall is important because the treatment modality and prognosis are accordingly different. A prospective study was performed to assess the value of transabdominal ultrasonography in the differentiation of early gastric cancer limited within the mucosa and submucosa from advanced cancer extending into the muscle layer. Fifteen patients with early gastric cancer and 29 patients with advanced gastric cancer were evaluated preoperatively and compared with histological findings. Patients were referred after endoscopy and sonologists were informed of the site of the gastric lesions. Patients ingested 600-800 ml of boiled water and transabdominal ultrasonography of the gastric lesion was accomplished using a commercially available ultrasound equipment with 5.0 and 3.5 MHz transducers. Using the intact middle hyperechoic layer on sonogram as the criterion for early cancer, 10 of 15 patients with early gastric cancer and 27 of 29 patients with advanced gastric cancer were correctly diagnosed (84%). Fourteen of 15 cases with early gastric cancer showed a thickness of the lesion below 1.0 cm, while seven of 29 cases with advanced cancer showed a thickness 1.0 cm or less. We believe that transabdominal sonography may be a valuable method in the differentiation between early and advanced gastric cancer.