Background and aim: Thrombocytosis is a marker of inflammatory reaction with varied clinical conditions. The significance of thrombocytosis in relation to severity of the disease in under-five children with lower respiratory tract infection was evaluated. Method: It was a prospective observational study conducted during 12 months on 230 children within ages, 2 to 59 months, hospitalized for lower respiratory tract infection. Admitted patients were classified as children with and without thrombocytosis. Based on clinical severity patients were grouped; complications from thrombocytosis were analyzed and compared with those without thrombocytosis. Results: Of 230 children, 70 (30.4%) patients were with pneumonia, 64 (27.82%) were with severe pneumonia and 96 (41.7%) were with very severe pneumonia. Severity of pneumonia was more in infancy and 53.04% of total patients with pneumonia had thrombocytosis. The association between total leucocytes count and severity of pneumonia was significant. Thrombocytosis was seen in 82% case of pneumonia with pleural effusion. There was significant association of thrombocytosis with prolonged hospital stay. The mortality was 2.2% which had no significant association with thrombocytosis. Conclusion: Thrombocytosis is a frequent finding among children with lower respiratory tract infection. Children with thrombocytosis have more severe pneumonia with longer duration of hospitalization than those with normal platelet count. Platelet count may be used as a marker of disease severity and complication in children with lower respiratory tract infection.