Although thrombocytosis is frequently detected in newborns, few reports have focused on its risk factors. This report documents the characteristics of 89 neonates with thrombocytosis followed up in a neonatal intensive care unit (NICU). We reviewed the patients' medical and laboratory records retrospectively to determine the associated conditions and risk factors for neonatal thrombocytosis, and complications related to thrombocytosis, We also discussed the differences of neonatal thrombocytosis from that of childhood in the light of literature, The mean platelet count of these newborns was 579.7 ± 111.5 (451-936) × 109/L. Associated conditions included anemia (73.7%), high-risk pregnancies (56. %), prematurity (51.7%), infections (37.1%), antenatal drug use (22.7%), indirect hyperbilirubinemia (20.2%), cardiac disorders (14.6%), respiratory distress syndrome (14.6%), history of hypoxia (13.5%), surgery (12.4%), and hemorrhage (3.4%). In most cases, more than one risk factor for thrombocytosis existed in the same newborn. No hemorrhagic complications related to thrombocytosis were observed; however, 1 newborn had portal vein thrombosis associated with intestinal malrotation, In summary, risk factors and associated conditions for thrombocytosis in newborns differed from those in children. In light of associated disorders, the mechanism of thrombocytosis in newborns may differ from that of childhood thrombocytosis. © Turkish Society of Hematology.