Aim: Supraventricular and ventricular extrasystole are the most common form of arrhythmia in paediatric cardiology, but still they may be confusing for paediatricians and family doctors. The aim of the study was to assess the scale of the problem based on the data from a single-centre department of paediatric cardiology. Materials and methods: This single-centre retrospective study analysed medical documentation of a total of 90 children hospitalised in the Department of Paediatric Cardiology due to suspected or diagnosed extrasystole from January 1st to December 31st 2019. Results: The study group of 90 children aged between 3 days and 17.8 years (57% male, mean age 11.1 ± 5.7 years) accounted for 4% of all hospitalisations. This was a new diagnosis in 35 (39%) patients, and 17 (19%) cases were referred as an emergency. Clinical symptoms were observed in 26 (29%) children. Extrasystole were registered in 74 patients (82%) using 24-hour Holter electrocardiographic monitoring. Premature ventricular extrasystole was the most common form of arrhythmia. Complex arrhythmias were registered in 18 (24%) patients. In 3 patients, arrhythmic burden exceeded 20%. Echocardiography showed no abnormalities in 67 (74%) children. A total of 11 children had a history of surgery due to congenital heart defects. In none of patients arrhythmia increased on exertion during treadmill test. A life-threatening cause of arrhythmia was detected in only 2 cases. A total of 18 patients required antiarrhythmic therapy. Conclusions: Supraventricular and ventricular extrasystoles are a common, but rarely life-threatening condition in children. It is important to establish the diagnosis and identify high-risk patients requiring treatment or close follow-up.