PURPOSE:This study was undertaken to identify prognostic factors that can be used to predict prognosis after surgery for lung metastases from colorectal carcinoma.METHODS:We reviewed retrospectively the clinical course of 37 patients who underwent surgical resection of primary colorectal cancer and metastatic lung disease at the National Defense Medical College Hospital between September 1986 and July 1999. We analyzed the prognostic factors with special reference to the clinicopathologic factors of primary tumors.RESULTS:Multivariate analysis indicated that the existence of an extranodal cancer deposit in the primary lesion (hazard ratio = 4.55,P= 0.009) and three or more lung metastases (hazard ratio = 2.9,P= 0.04) were significant indicators for poor prognosis. We divided the patients into two groups: Group A (n = 12) had neither of these two parameters, and Group B (n = 25) comprised all other patients. This two‐ranked classification was significantly related to both survival rates (3‐year and 5‐year survival rate, 90.9 and 90.9 percent in Group A and 16.1 and 8.1 percent in Group B, respectively;P= 0.0003) and disease‐free survival after thoracotomy (3‐year and 5‐year disease‐free survival rate, 52.9 and 39.7 percent in Group A and 5.3 and 5.3 percent in Group B, respectively;P= 0.002).CONCLUSION:An extranodal cancer deposit at the primary tumor site is a new significant prognostic factor after resection of lung metastases from colorectal cancer. A two‐ranked classification by extranodal cancer deposit and the number of pulmonary lesions can provide useful prognostic information for the treatment of lung metastasis. Surgical resection of pulmonary metastasis is expected to be very useful for patients in Group A.