The prognostic value of histologic classifications of gastric adenocarcinoma is controversial, although they have been commonly used. The clinical significance of the mucin phenotype has not been clarified. This study was conducted to determine the clinical significance of mucin phenotype as a possible prognostic factor. Mucin histochemistry by paradoxical concanavalin A (Con A) staining and immunostaining for 45M1, MUC2 glycoprotein and CD10 of mucin was performed in surgically obtained paraffin-embedded specimens from 106 gastric adenocarcinomas. We determined their mucin phenotypes and analyzed their relationships with clinical and histopathologic variables and survival rates. Among 106 gastric adenocarcinomas, 37 (34.9%), 35 (33.0%), 22 (20.8%) and 12 (11.3%) expressed the intestinal (I-), the gastric (G-), mixed (M-), and undetermined (U-) phenotypes, respectively. Although the mucin phenotype correlated well with histologic differentiation (p=0.000) and Lauren's classification of a tumor (p=0.003), it did not accord completely with them. There was no relationship between mucin phenotype and other patient clinicopathologic variables. No statistically significant difference in survival was observed among mucin phenotypes on univariate (p=0.089) and multivariate (p=0.088) analyses. However, the patients with I-phenotype tumor had a significantly better outcome than those with non-I-phenotype tumor on univariate (p=0.023) and multivariate (p=0.049) analyses. In conclusion, the mucin phenotype did not accord completely with histologic differentiation and Lauren's classification of gastric adenocarcinoma, despite a well-defined correlation between them. I-phenotypic expression, but not the histologic differentiation and Lauren's classification, was found to be an independent good prognostic factor of gastric cancers.