Purpose: Croup is common respiratory illness in children which accounts for approximately 15% of lower respiratory tract infections. The importance of croup in lower respiratory tract infections can not be ignored, but comprehensive studies such as epidemiological studies on croup are insufficient in Korea. This study aims to analyze the age, sex, etiology and seasonal variation of respiratory virus infections in children diagnosed with croup through long-term study from 2009 to 2017. Methods: We examined the patients under 18, hospitalized with croup at Gachon University Gil Hospital from February 2009 to April 2017. Medical records of children with age, sex, diagnosis, admission date and respiratory virus test result were reviewed retrospectively. Results: A total of 1050(7.3%) of 14366 infected patients were diagnosed with croup. Males (60.95%) were more common than females. For age distribution, age between 12 to 24 months was most frequent (50.29%), and more than 85% were under 36 months. There were two peaks, in summer (July to August) and spring (April to May). Winter(december) had the lowest frequency. 60.95% of children admitted with croup were tested for respiratory virus. Virus were identified in 85.94%. Parainfluenza virus type 1 (16.36%) was the most common virus, followed by RSV A (8.18%) and rhino virus (8.18%). The number of cases that had two or more viruses were detected reached 28.73%. Conclusions: In this study, we could see the seasonal fluctuation of croup with two peaks and one trough. Parainfluenza virus type 1 was the most frequently detected virus. It is believed that by obtaining epidemiologic data of croup which is an important part of the children who are hospitalized or visiting the outpatient clinic, basic data for croup care can be provided that helps medical treatment