In healthy pediatric subjects normal count platelet ranges between 250,000 μL and 450,000 μL. An elevated platelet count greater than 2 SD defines a condition of thrombocytosis. On a clinical level, thrombocytosis is classified "mild" at a platelet count between >500,000 μL and <700,000 μL; "moderate" at a platelet count between >700,000/μL and <900,000/μL; "severe" at a platelet count >900,000/μL; and "extreme" at a platelet count >1,000,000/μL. Thrombocytosis can be classified as primary or secondary. Primary thrombocytosis is divided into familial and essential. Primary thrombocytosis is an extremely rare clonal disease in childhood with incidence of one per million children, i.e., 60 times lower than in adults. It is classified as a myeloproliferative disorder with polycythemia vera, chronic myeloid leukaemia and myelofibrosis and may be associated with thrombotic or haemorrhagic events. Platelet count is generally above 1,000,000/μL. The median age at diagnosis is about 11 years. On the contrary, secondary or reactive thrombocytosis (RT) is very common in pediatric age, occurring in 3-13% of hospitalized children because of several causes. Generally, it is a reactive process caused by infection, chronic inflammation, iron deficiency, tissue damage, cancer, drugs and surgical or functional splenectomy. Thrombocytosis is mild in 72-86% of children, moderate in about 6-8%, and extreme in 0.5-3%. Consultation with a pediatric hematologist is required if elevation of platelet count persists, is unexplained or symptomatic. In the majority of cases no treatment is necessary, and the patient must be only closely monitored.