PURPOSE: Thrombocytosis is commonly associated with various tumors, including stomach, ovarian, lung, liver and pancreas cancers. Some clinical reports have shown thrombocytosis to be associated with the disease stage and prognosis. This study investigated the prevalence of the thromobocytosis in patients with colorectal cancer, and its association with the prognosis. METHODS: Two hundreds ninety-six patients with colorectal cancer who had been surgically treated at our hospital between 1997 and 2004 were enrolled in this study. The incidence, relationship with other clinicopathological factors, and the prognostic value of thrombocytosis were evaluated retrospectively. RESULTS: Thirty-seven of the 290 (12.8%) patients had thrombocytosis. The incidence of thrombocytosis was examined with regard to gender (P=0.018), tumor location (P=0.021), and T stage of tumor (P<0.000). Univariate analysis demonstrated thromobocytosis, T stage, N stage, M stage, venous invasion, lymphatic invasion, and elevated carcinoembryogenic antigen (> or =5 ng/ml) to be associated with both the disease-free survival (DFS) and overall survival (OS). Multivariate analysis revealed thrombocytosis to be significantly associated with the disease-free survival (P=0.026). CONCLUSION: Preoperative thrombocytosis appears to be an independent prognostic indicator of the DFS in patients with colorectal cancer.