Background: Thrombocythemia (ET) is defined as elevation in the platelets count more than 400,000/ µ L in peripheral smear. It is common in infant and children that occurs in 3 to 13% of children. Thrombocytosis is classified into primary/clonal or secondary according to the pathological origin. Objective: To determine incidence, causes and types of thrombocytosis in children also the correlations between the severity of thrombocytosis and the etiology and platelets indices. Patients and methods: Prospective, observational study was done in Pediatric Department and Pediatric Out-Patient Clinic, Sohag University Hospital. A total of 117 cases of thrombocytosis were included in this study. All children had thrombocytosis and aged from one month to twelve years old. Results: Reactive thrombocytosis was observed in 114 cases (97.44%), while primary type was found only in 3 cases (2.56%), 2 of them had chronic myelogenouse leukemia and one case is diagnosed as essential thrombocytosis. Majority of children (96 cases) in our study patients had mild thrombocytosis (82.05%) while moderate thrombocytosis was found in 16 cases and severe thrombocytosis were seen in 5 cases with 13.67% and 4.27% of children respectively. The correlation between platelet number and the mean platelet volume (MPV) showed significant negative correlation (p < 0.001), while the correlation between the platelet number and plateletcrit (PCT) showed significant positive correlation (p < 0.001) but platelet distribution width (PDW) had no correlation with platelet number (P= 0.7). Conclusion: Among our study platelet indices, the MPV had a significant negative correlation with platelet count and MPV was considered normal in patients with reactive thrombocytosis.