Proteolytic activity of cancer cells is an important factor in metastasis. This study examined the relationship between MMP-1 expression of gastric cancer cells and peritoneal metastasis. MMP-1 expression was found in 76/103 (75.2%) cases examined and was significantly associated with both peritoneal metastasis and lymph node metastasis (p<0.05, respectively). The prognosis of patients with MMP-1 positive tumor was significantly worse than that of patients with MMP-1 negative tumor (p<0.05). These findings suggested that MMP-1 might be a prognostic factor in case of advanced gastric cancer and might be useful in determining whether or not adjuvant therapy was indicated for patients at high risk of peritoneal recurrence.