In patients with thrombocytosis normal to increased serum thrombopoietin (TPO) levels have been reported. The aim of this study was to investigate the relationship between serum TPO concentration, platelet number and plasma levels of fibrinogen in patients with reactive thrombocytosis (RT) due to lung cancer and in patients with essential (primary) thrombocythemia (ET). A total of 70 newly diagnosed patients with RT or ET (platelet counts >600 G/l) were studied: 45 with RT due to lung cancer (25 with non-small cell lung cancer, NSCLC and 20 with small cell lung cancer, SCLC), and 25 with ET. Twenty normal volunteers were used as controls. TPO was measured by immunoassay technique (ELISA). Mean serum TPO values in patients with RT due to lung cancer were statistically significantly higher than those in patients with ET or in controls. The highest platelet count was seen in patients with ET, and mean plasma fibrinogen levels were the highest in RT patients. In NSCLC patients mean serum TPO concentrations were higher (not statistically significant) than in SCLC, and a statistically significant relationship between TPO serum concentration and fibrinogen level was observed. No correlations between platelet counts and TPO serum concentrations were found. Our results indicate increased serum TPO levels in patients with thrombocytosis in lung cancer which may be related to the activity of neoplasms. In addition, it is postulated that the relatively low TPO values in patients with ET may result from a dysregulation of the feedback loop involved in platelet production.