The clinicopathological features of 84 muscularis propria (mp) gastric cancers, defined as gastric cancer invading the muscularis propria of the stomach, were reviewed retrospectively, and compared with 267 early gastric cancers (m/sm cancer) and 333 gastric cancers invading beyond the subserosal layer (ss-si cancer). There were statistical differences in tumor size, histological growth pattern, cancer-stromal relationship, lymph node metastasis, lymphatic invasion, vascular permeation and operative procedure between mp gastric cancer and early gastric cancer or ss-si gastric cancer. The mp tumors were significantly larger than the early cancer tumors but significantly smaller than the ss-si tumors (P < 0.0001), and mp cancer had more frequent lymph node metastasis (50%) than did early gastric cancer (10%) but less frequent lymph node metastasis than did ss-si cancer (84%; P < 0.0001). Histologically, infiltrative and scirrhous types were more common in the mp cancer group than in the early cancer group. The frequency of vascular permeation in the mp cancer group was almost the same as that in the early cancer group. Univariate analysis revealed that the significant prognostic factors were nodal involvement (P = 0.0213) and lymphatic invasion (P = 0.0364). Multivariate analysis of the mp-invaded cancer cases, however, revealed that lymph node metastasis was not a significant prognostic factor, although it was more important than was lymphatic invasion. Multivariate analysis also revealed that the prognosis of our mp gastric cancer patients was affected most by vascular permeation, followed by tumor size.