Although the major histologic type in small gastric cancers, less than 10 mm in diameter, is differentiated-type adenocarcinoma (D.Ca), the incidence of D.Ca and that of undifferentiated-type adenocarcinoma (UD.Ca) is almost the same in all early gastric cancers. Histologic conversion from D.Ca to UD.Ca has been speculated, however, a detailed examination of this phenomenon has not yet been performed. Three-hundred and 51 early gastric cancers (D.Ca, 150 (42.7%) lesions; UD.Ca, 93 (26.4%) lesions; and mixed differentiated and undifferentiated type (D&UD.Ca), 108 (30.8%) lesions; tumor size less than 10 mm in diameter; 64 lesions, more than 10 mm, 287 lesions) were examined histochemically with paradoxical concanavalin A type III and high-iron diamine-Alcian blue (pH 2.5), and immunohistochemically with antigastric mucin antibody. The associations between tumor size, tumor differentiation and phenotypic expression of mucin were examined. Regardless of the tumor size, mucin phenotypic expression in the mucosal lesions examined was preserved. Of 47 cancers with a gastrointestinal mucin phenotype (GIM type) or a gastric mucin phenotype (GM type) measuring less than 10 mm, 35 (74.5%) consisted of D.Ca and 12 (25.5%) of both D&UD.Ca and UD.Ca, while of 224 GIM or GM type cancers measuring more than 10 mm, 64 (28.6%) consisted of D.Ca and 160 (71.4%) of both D&UD.Ca and UD.Ca. Differences between these two groups were statistically significant (P < 0.001). Of 15 cancers with an intestinal mucin phenotype (IM type) measuring less than 10 mm, 12 (80.0%) consisted of D.Ca and three (20.0%) of both D&UD.Ca and UD.Ca, and of 50 IM type cancers measuring more than 10 mm, 35 (70.0%) consisted of D.Ca and 15 (30.0%) of both D&UD.Ca and UD.Ca. Differences between these two groups were not statistically significant. These findings suggest that small D.Ca showing gastric mucin expression may transform into UD.Ca during the progression of early gastric cancer.