AIM To determine the true prevalence of thrombocytosis in children less than 2 years of age with bronchiolitis,its association with risk factors,disease severity and thromboembolic complications.METHODS A retrospective observational medical chart review of 305 infants aged two years or less hospitalized for bronchiolitis.Clinical outcomes included disease severity,duration of hospital stay,admission to pediatric intensive care unit,or death.They also included complications of thrombocytosis,including thromboembolic complications such as cerebrovascular accident,acute coronary syndrome,deep venous thrombosis,pulmonary embolus,mesenteric thrombosis and arterial thrombosis and also hemorrhagic complications such as bleeding(spontaneous hemorrhage in the skin,mucous membranes,gastrointestinal,respiratory,or genitourinary tracts).RESULTS The median age was 4.7 mo and 179 were males(59%).Respiratory syncytial virus was isolated in 268(84%),adenovirus in 23(7%) and influenza virus A or B in 13(4%).Thrombocytosis(platelet count > 500 × 109/L) occurred in 88(29%;95%CI:24%-34%),more commonly in younger infants with the platelet count declining with age.There was no significant association with the duration of illness,temperature on admission,white blood cell count,serum C-reactive protein concentration,length of hospital stay or admission to the intensive care unit.No death,thrombotic or hemorrhagic events occurred.CONCLUSION Thrombocytosis is common in children under two years of age admitted with bronchiolitis.It is not associated with disease severity or thromboembolic complications. AIM To determine the true prevalence of thrombocytosis in children less than 2 years of age with bronchiolitis, its association with risk factors, disease severity and thromboembolic complications. METHODS A retrospective observational medical chart review of 305 infants aged two years or less hospitalized for bronchiolitis. Clinical outcomes included disease severity, duration of hospital stay, admission to pediatric intensive care unit, or death. They also included complications of thrombocytosis, including thromboembolic complications such as cerebrovascular accident, acute coronary syndrome, deep venous thrombosis, pulmonary embolus, mesenteric thrombosis and arterial thrombosis and also hemorrhagic complications such as bleeding (spontaneous hemorrhage in the skin, mucous membranes, gastrointestinal, respiratory, or genitourinary tracts) .RESULTS The median age was 4.7 mo and 179 were males (59%). Respiratory syncytial virus was isolated in 268 (84%), adenovirus in 23 (7%) and influenza virus A or B in 1 Thrombocytosis (platelet count> 500 × 109 / L) occurred in 88 (29%; 95% CI: 24% -34%), more commonly in younger infants with the platelet count declining with age. no significant association with the duration of illness, temperature on admission, white blood cell count, serum C-reactive protein concentration, length of hospital stay or admission to the intensive care unit. No death, thrombotic or hemorrhagic events occurred. in children under two years of age admitted with bronchiolitis. It is not associated with disease severity or thromboembolic complications.