Objective: The goal of the study was to assess the incidence of arrhythmias in children with mitral valve prolapse (MVP). Methods: The study involved 67 children with MVP 47 girls, 20 boys, aged 8-18 years (average 13,9 ±2,61 years). Control group consisted of 31 healthy children matched according to age and sex. In all children ECG, 24-hour Holter ECG monitoring and treadmill test according to Bruce protocol were performed. Results: ECG showed arrhythmia in 7 (10,4%) children with MVP. In 24-hour Holter ECG monitoring arrhythmia was detected in 19 (28,3%) patients, including 8 (11,9%) patients with ventricular and 11 (16,4%) with supraventricular arrhytmia. Ventricular arrhythmia was classified according to Lown scale as I grade in 1 case, II grade in 3 patients and IV grade in 4 cases. Complex atrial arrhythmias including atrial tachycardia and atrial couplets were present in 4 (5,9%) patients. 8 (11,9%) children with MVP had positive treadmill test result due to arrhythmia: in 6 children because of arrhythmia induced by exercise (with presyncopal stage in one patient), in 2 because of arrhythmia exacerbation. Complete suppression of arrhythmia during treadmill test was observed in 6 (8,9%) children and in one (1,5%) child the effort had no influence on arrhythmia. Conclusions: 1/ In children with mitral valve prolapse arrhythmia is detected in about 30% cases. 2/ Physical activity should be restricted only in patients, with positive result of treadmill test due to new rhythm disturbances or exacerbation of arrhythmia.