We examined immunohistochemically the expression of the standard form of CD44 (CD44S) tissue specimens from 164 patients with pulmonary adenocarcinoma. Of the 164 specimens, 79 (48%) expressed CD44S and the incidence of expression correlated with the tumor size. The prognosis for CD44S positive patients was poorer than that of CD44S negative patients. Our findings suggest that CD44S plays an important role in tumor progression and that CD44S expression is a useful prognostic marker for pulmonary adenocarcinomas.