OBJECTIVE: The objective of this study was to investigate the frequency of secondary thrombocytosis in children during RSV bronchiolitis and bronchiolitis caused by some other cause. Predictive value of secondary thrombocytosis in relation with severity of clinical features and the outcome of the treatment in bronchiolitis affected children has also been studied. STUDY DESIGN: Retrospective study. PARTICIPANTS AND METHODS: Medical records of 136 infants treated at Pediatrics department of County General Hospital Požega from January first, 2012 to January first, 2017 were used in this study. Demographic and other patients' features with bronchiolitis were collected from their medical records. The data was statistically examined in SPSS for Windows (version 16.0, SPSS Inc., Chicago, IL, SAD). RESULTS: The statistically significant association between duration of hospitalization and oxygen application in therapy of thrombocytosis was determined with logistic regression. The risk for thrombocytosis was increased with every day of hospitalization by 18,5 %, and for the oxygen application in therapy by 33,1 %. Need for hospitalization and development of pneumonia statistically do not have an influence on the development of thrombocytosis. CONCLUSION: Children with bronchiolitis and thrombocytosis had a more severe course of the disease, which was presented with longer duration of hospitalization and the need for oxygen application in therapy.