PURPOSE: Current diagnostic modalities frequently carry false negative evaluations, especially in micro-metastasis. Some metastases are identified incidentally during the operation for primary colorectal cancer. Our study was performed to assess the clinicopathological characteristics of intraoperatively diagnosed liver metastases, to analyze the survival and the prognosis, to compare the results with those for preoperatively diagnosed liver metastases. METHODS: Between July 1989 and December 2001, the cases of 78 patients who underwent treatment for intraoperatively diagnosed liver metastasis at our hospital were analyzed retrospectively. There were 375 patients who were diagnosed with liver metastasis preoperatively during the same period. Metachronous liver metastases were excluded. RESULTS: Intraoperatively diagnosed liver metastases mostly showed clinicopathological characteristics of primary colorectal cancer similar to those for preoperatively diagnosed liver metastases. On the other hand, the preoperative serum CEA level was significantly lower in the intraoperatively diagnosed group than it was in the preoperatively diagnosed group (P<0.001). For the metastatic lesion, the size of the metastasis was smaller in the intraoperatively diagnosed group than it was in the preoperatively diagnosed group (P=0.03). The two-year survival rate of the intraoperatively diagnosed group was significantly better than that of the preoperatively diagnosed group (71.6% vs. 58.4%; P=0.031). Prognostic factors of the intraoperatively diagnosed group were the number of liver metastases, a curative operation for the primary cancer, and an operation for the metastatic lesion. CONCLUSIONS: Intraoperatively diagnosed liver metastases had biologic features similar to those of preoperatively diagnosed liver metastases. The survival rate of the intraoperatively diagnosed group was better than that of the preoperatively diagnosed group, possibly due to the curative resection for the hepatic metastases. Therefore, aggressive treatment for primary and metastatic lesions is the therapeutic choice to improve patient's survival for intraoperatively diagnosed synchronous liver metastases.