Thrombocytosis, or elevations of platelet count in the peripheral blood to values >400,000/mm3, is common in infancy and childhood, occurring in 3-13% of children. Primary thrombocytosis is divided into familial & essential thrombocytosis, while reactive thrombocytosis occurs frequently. Most of the available studies on the thrombocytosis in children are from developed countries, and there appears to be geographical variations in causes of reactive thrombocytosis (RT). The present study was undertaken to ascertain common etiology of reactive thrombocytosis, to assist the general pediatrician in dealing with this common hematological finding in everyday clinical practice. This study was conducted on 100 inpatients admitted under the department of pediatrics, Manipal hospital, Bangalore from August 2011 to July 2012. Consecutive children of both sexes up to the age of 5 years, having thrombocytosis (platelet count > 4,00,000/mm3) were enrolled. An appropriate EDTA blood sample was collected at admission and platelet counts were analysed by automated analyser (Beckman coulter 780) and reconfirmed by manual method. Relevant investigations were done according to the clinical history and physical findings, to establish the diagnosis. Age, sex and varied diagnosis were correlated with the thrombocytosis and results were analysed. Reactive thrombocytosis is a very common observed condition in children, unlike primary thrombocytosis which is rare. Most of the literature is from the western world, and this study was conducted to know the common etiologies responsible for reactive thrombocytosis in this part of the country. In most children with reactive thrombocytosis, platelet counts are modestly elevated up to 700,