Introduction and objective:
Pneumonia is defined as an acute infection of the lung parenchyma caused by various pathogens. It remains a leading cause of morbidity and mortality in the paediatric population globally. The aim of this study was to disclose most common aetiology of pneumonia among children hospitalized in the University Children’s Hospital in Lublin, Poland, from 2010–2020.

Material and methods:
This retrospective study included 2,250 patients in the age rage from 2–227 months, hospitalized in the Department of Paediatric Pulmonology and Rheumatology of University Children’s Hospital in Lublin from 2010–2020 due to pneumonia. All necessary information were obtained from electronic medical records. According to the International Classification of Diseases Version 10 (ICD-10), patients were divided into 20 groups in terms of aetiology of pneumonia. Subsequently collected data were submitted to statistical calculations.

Results:
A seasonality of pneumonia admissions was observed with predominance in the winter. Based on ICD-10 codes, the most common aetiology of pneumonia was unspecified bacterial pneumonia (J15.9) related with 30.2% of all cases. Moreover, based on IgM serology tests, Mycoplasma pneumoniae was the main identified pathogen (18.8% of all cases). Average duration of hospitalization oscillated around 7.4 days; the most common administered antibiotic was amoxicillin + clavulanic acid. Chest imaging was performed in 88.8% of patients.

Conclusions:
Pneumonia dominated in the bacterial aetiology of children. Younger groups of children were more prone to the development of pneumonia. In many casus, there were observed difficulties with the identification of an exact pathogen.