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Isolated tumor cells are often found in the regional lymph nodes of colorectal cancer, although their prognostic significance has not been established yet. This study was performed to investigate the correlation between the presence of isolated tumor cells in lymph nodes and the histopathologic characteristics of colorectal cancers and, thus, to determine which factors are associated with isolated tumor cells.We used immunohistochemistry with anticytokeratin antibody to examine 2,784 lymph nodes in 109 patients with node-negative colorectal cancers. The clinicopathologic features of the tumors with isolated tumor cells were compared with those without isolated tumor cells. The frequency, number, and level of the isolated tumor cells also were assessed.Isolated tumor cells were detected in 335 lymph nodes (12 percent) from 71 patients (65.1 percent). Those tumors having isolated tumor cells in lymph nodes, compared with those not having isolated tumor cells, were characterized by large tumor size, high T stage (pT3 and pT4), angiolymphatic invasion, perineural invasion, absence of peritumoral lymphocytic response, microsatellite instability-negative phenotype, and tumor budding. Multivariate analysis showed that those factors independently associated with the presence of isolated tumor cells were high T stage, tumor budding, and microsatellite instability-negative phenotype. Among the 71 patients with high T stage and microsatellite instability-negative phenotype, tumors with isolated tumor cells were characterized by a high frequency of tumor budding compared with tumors without isolated tumor cells (85vs.36.4 percent). In a further study, the degree of budding, which was assessed by an immunohistochemical study of γ2 chain of laminin-5, was closely related to the number and location of isolated tumor cells. Moreover, we found that most of the isolated tumor cells in the regional lymph nodes also expressed γ2 chain of laminin-5.Our results suggested that isolated tumor cells are derived from undifferentiated cancer cells or small clusters (“budding”) at the invasive front. Thus, tumor budding may be used as an indicator of isolated tumor cells in lymph nodes with node-negative colorectal cancers.