Invasive procedures involving the atria may promote the development of iatrogenic cardiac arrhythmias. To analyse the prevalence of cardiac arrhythmias following transcatheter or cardiosurgical closure of the secundum type atrial septal defect (ASD). The study group consisted of 91 patients, aged 2-18 years with haemodynamically significant ASD who underwent surgical (n=44) or transcatheter (Amplatzer occluder) (n=47) closure of ASD. Standard ECG and Holter ECG recordings, obtained before and after the procedure, were analysed. The follow-up duration ranged from 2.5 to 5.5 years. Cardiac arrhythmias were divided into benign or significant (requiring pharmacological therapy), early or late, and transient or permanent. Cardiac arrhythmias were detected in 16 (36%) patients who underwent surgery compared with 1 (2.1%) patient who underwent transcatheter ASD closure (p<0.05). In surgically treated patients, arrhythmias were benign in 9 patients, significant in 7 children, early in 15 subjects, late in one patient, transient in 13 children and permanent in 3 subjects. One patient, who underwent transcatheter ASD closure, developed paroxysmal supraventricular tachycardia one day after the procedure, successfully terminated with verapamil. Transcatheter closure of ASD is associated with a lower risk of procedure-related arrhythmias than surgical treatment. However, longer follow-up in patients treated with transcatheter procedure is needed in order to draw definite conclusions.