Several clinical studies have shown that thrombocytosis is a poor prognostic factor in some types of cancer, but data about the impact of thrombocytosis on prognosis in patients with colon cancer are very limited. We investigated the prevalence and prognostic effect of pre-operative thrombocytosis, defined as a platelet count > 400 x 10(9)/l, retrospectively in patients with node-negative colon cancer. Out of 198 patients, 24 (12.1%) had thrombocytosis, and its presence correlated with tumour depth and lymphatic invasion. Univariate analysis revealed that disease-free survival and overall survival were shorter in patients with pre-operative thrombocytosis than those without thrombocytosis. On multivariate analysis, thrombocytosis alone retained significance as a poor prognostic factor for both disease-free survival and overall survival. In conclusion, this study shows an association between thrombocytosis and poor survival in patients with node-negative colon cancer. The preoperative platelet count may help to identify patients with an unfavourable prognosis in this subgroup.