AIM: Hepatocellular carcinoma (HCC) patients manifest a variety of paraneoplastic syndromes. Thrombocytosis was reported in children with hepatoblastoma. The aims of this study were to evaluate the prevalence and dinical significance of thrombocytosis in HCC patients and its relationships with serum thrombopoietin (TPO).METHODS: We retrospectively reviewed clinical, biochemical and image data of 1 154 HCC patients. In addition, we measured platelet count and serum TPO in HCC patients with and without thrombocytosis, in patients with cirrhosis,chronic hepatitis and healthy subjects in a cross-sectional study.RESULTS: Thirty-one (2.7%) of 1 154 HCC patients had thrombocytosis (platelet count ≥400 K/mm3). HCC patients with thrombocytosis were significantly younger, had a higher serum α-fetoprotein, higher rate of main portal vein thrombosis, larger tumor volume, shorter survival, and were less likely to receive therapy than HCC patients without thrombocytosis. Multivariate logistic regression analyses showed that tumor volumes ≥30% and serum α-fetoprotein ≥ 140 000 ng/mL could significantly predict thrombocytosis.HCC patients with thrombocytosis had a significantly higher mean serum TPO than those without, as well as patients with cirrhosis, chronic hepatitis and healthy subjects.Platelet count and serum TPO dropped significantly after tumor resection in HCC patients with thrombocytosis and re-elevated after tumor recurred. Furthermore, the expression of TPO mRNA was found to be more in tumor tissues than in non-tumor tissues of liver in an HCC patient with thrombocytosis.CONCLUSION: Thrombocytosis is a paraneoplastic syndrome of HCC patients due to the overproduction of TPO by HCC.It is frequently associated with a large tumor volume and high serum α-fetoprotein.