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Background. Sialyl Lewisa antigen (SLA) is considered to be a cancer-associated carbohydrate antigen. Methods. A total of 309 surgically resected primary colorectal cancer specimens and 501 associated metastases to regional lymph nodes were analyzed immunohistochemically using a monoclonal antibody (NS19-9) recognizing SLA. The specimens were obtained from 1981 to 1988 at the Department of Surgery, Keio University 1 School of Medicine (Tokyo, Japan). Results. Sialyl Lewisa antigen expression was detected in 75.4% (233/309) of the primary tumors and in 78.5% (99/126) of the metastases to regional lymph nodes. Significantly stronger expression was noted in the regional lymph node metastases than in the primary lesions (P = 4.5E−7). Sialyl Lewisa antigen expression in the primary tumors correlated significantly with regional lymph node metastasis (P < 0.005), recurrence (P < 0.005), S and postoperative survival (P < 0.001) as determined by univariate analysis. Sialyl Lewisa antigen expression in the regional lymph node metastases also correlated strongly with recurrence (P < 0.005) and survival (P < 0.01). As determined by multivariate analysis, SLA expression in the primary tumor correlated strongly with recurrence (P = 3.0E−6) and survival (P = 6.9E−3). Conclusions. Sialyl Lewisa antigen expression in primary tumors and metastases to regional lymph nodes is a useful marker for evaluating tumor aggressiveness and prognosis in patients with advanced colorectal cancer.