Objectives. Our primary purpose was to establish the prevalence of the diseases which could be associated with thrombocytosis in children. Methods. It is a transversal study in patients less than fifteen years old in children's hospital "Lorencita Villegas de Santos". Clinical files were selected and rewied under convenience criteria: patients with platelet count higher than 450 * 1O/L,if they had in addition a total leucocyte count and an eritrocyte sedimentation rate, and a diagnosis of an illness in which physicians based treatment. Patients with splenectomy were excluded. Descriptive and correlational statistics were applied. Results. Diseases associated with thrombocytosis were, in decreasing order of prevalence: infectious deseases (62.5%), malignant diseases (lO.4 %), most of them lymphoreticular and hematological origins, inflammatory diseases (5%), specifically bronchial asthma and allergic rhinitis. No differences were observed between groups of diseases and platelet count. Older mean age was found in patients with malignant diseases, following by infectious diseases; difference was statistically significant. No correlation was observed between platelet and white cell counts. A weak lineal positive correlation between platelet count and eritrocyte sedimentation rate was statistically significant. Conclusions: In children and infants (between 0 and 13 years of age) thrombocytosis was associated, in decreasing order of prevalence, with: infectious diseases, cancer of Iymphoreticular and hematological origins and inflammatory diseases, specially in those of allergic cause. Mean age of patients seems to be older in patients with malignant diseases compared with other groups studied. No difference was found in platelet count between the groups studied; therefore,reactive thrombocytosis is unable itself to suggest the underlying disease.