Thrombosis is one of the most important complications after splenectomy and requires fast diagnosis, effective therapy and good follow-up. The aim of this study is to investigate the effects of thrombocytosis and natural inhibitors on thrombosis after splenectomy. We detected thrombosis in the portal vein system in 7 of the 30 splenectomized patients (23.3%) by Doppler Colour Flow Imaging. There was no statistical increase of thrombocyte count in patients with or without thrombosis. Natural inhibitor levels in all patients were lower than controls (p< 0.001), but there was not any statistical difference between the patients with and without thrombosis.