Objective. To study the features of the clinical duration of atrial fibrillation (AF) and the structure of concomitant cardiac arrhythmias in children. Patients and methods. The pilot retrospective controlled non-randomized two-center study included 39 children aged 3 to 17 years (tmain group) who were on inpatient treatment at the Research Clinical Institute of Pediatrics and Pediatric Surgery academician Yu.E.Veltischev in the period from 2010 to 2020. The comparison group consisted of 156 patients aged 29 to 77 years suffering from paroxysmal and persistent forms of AF, who underwent interventional treatment from 2013 to 2021 at the V.M. Buyanov City Clinical Hospital of Moscow City Health Department and the National Medical Research Endocrinology Center of the Ministry of Health of Russia. Statistical processing was performed using STATISTICA application package. Quantitative features are represented by minimum and maximum values, as well as median (Me) and interquartile range (Q1 – 25 percent quartile, Q3 – 75 percent, respectively). The criteria of Fisher, Mann–Whitney, Pearson, and Spearman were calculated. The differences were considered reliable at p < 0.05. Results. The paroxysmal form of disease was diagnosed in 24 children (61.5%) cases, in 121 adults (77.6%). Clinical manifestations of AF in children were significantly more often recorded in its paroxysmal form – in 23 (95.8%) cases versus 8 (53.3%). In the comparison group, regardless of the form of AF, all patients had severe symptoms; the severity of clinical manifestations was founded in 93 (59.6%) cases, according to the classification of the European Heart Rhythm Association, corresponding to class III and above. In the structure of concomitant cardiac arrhythmias in patients of the main group, typical atrial flutter was in 13 (33.3%) cases, and atrial extrasystole -in 9 (23.1%). Bradyarrhythmias and conduction disorders (sinus node weakness syndrome, and atrioventricular blockade of I–II degrees) were founded in 10 (25.6%) children and only in 6 (3.8%) patients from the comparison group. Antiarrhythmic therapy in the main group was received by 30 (76.9%) children; in 28 (71.8%) cases – monotherapy with antiarrhythmic drugs of IC, II and III classes; all patients in the comparison group received IC and Class III preparations. Interventional treatment was performed in 6 (15.4%) patients from the main group and all from the comparison group. Conclusion. During diagnosing AF in children we should know the possibility of a frequent asymptomatic duration of disease, as well as complaints which are not typical for tachysystolic cardiac arrhythmias, isolated or in combination with typical. Clinical manifestations were significantly more often founded in paroxysmal AF in children. In the structure of concomitant AF cardiac arrhythmias in children, as well as in adult patients, atrial flutter prevails, and atrial extrasystole in children was recorded significantly more often, as well as sinus node weakness syndrome and atrioventricular blockade of I–II degree. The results of the study confirm both the need for a personalized approach to the diagnosis and treatment of AF in all age groups, and the creation of Guidelines for children and interventional treatment of this variant of arrhythmia. Key words: antiarrhythmic therapy, children, clinical duration, concomitant cardiac arrhythmias, atrial fibrillation