CD44, an integral membrane glycoprotein expressed by many cell types, serves as the principal transmembrane hyaluronate receptor and may be a determinant of metastatic and invasive behavior in carcinomas. The expression of CD44 in 23 gastric adenocarcinoma and 12 peptic ulcer disease (PUD) resection specimens and gastric carcinoma cell lines HS746t and KATO III was examined by immunohistochemistry using the murine monoclonal antibody A3D8 on formalin-fixed, paraffin-embedded tissue or cells. Western blot analysis of whole cell lysates of KATO III and HS746t cells showed protein bands at 85 to 90 kd with KATO III cells expressing an additional band at 145 kd. In normal stomach gastric epithelium was negative. In PUD foveolar epithelium was focally positive, but staining did not correlate with the extent of gastritis. In carcinoma cases intensity of staining was progressively stronger comparing intestinal metaplasia with dysplasia with intramucosal carcinoma. Invasive carcinoma was invariably more strongly positive than dysplasia or intramucosal carcinoma. Twelve adenocarcinomas were weakly positive and 11 were strongly positive. The staining intensity of metastases (12 cases) was the same or weaker than the primary tumor. For the 12 patients whose carcinomas were weakly positive, mean length of survival for the six who died was 23.3 months. Five of the 11 patients whose carcinomas strongly expressed CD44 died within the study period with a mean length of survival of 11.0 months. A key consequence of CD44 overexpression in gastric carcinomas may be development of the invasive phenotype and strong expression may indicate a poorer prognosis.