The platelet count above than 450.000 mm3 is defined as thrombocytosis. According to the underlying causes, thrombocytosis is classified as primary (essential) or secondary (reactive) thrombocytosis. In childhood, the most common causes of thrombocytosis are secondary reasons. Secondary thrombcytosis is a self-limited disease. It resolves with under the control of underlying disorder. Secondary trombocytosis usually does not result in tromboembolic event. If trombembolic complication occurs, such cases have got additional risk factors. Primary trombocytosis is grouped under the myeloproliferative disease and is exremely rare in childhood. Tromboembolic events in primary trombocytosis are the most important causes of mortality and morbidity. Some cases with primary thrombocytosis may present with bleeding symptoms.