The frequency of arrhythmias was determined pre- and postoperatively in adult patients with secundum atrial septal defect. The study group consisted of 224 pts who had undergone surgical repair of ASD II in between 1987-1993. The mean postoperative follow-up was 7.5 y. The group (160 F, 64 M, aged 17-66 y) was divided on 3 subgroups: I--age under 20 y--27 pts, II--20-40 y--131 pts, III--over 40 y--66 pts. We analysed pre- and postoperative parameters: 1) arrhythmias in the 24-h Holter recording, 2) pulmonary hypertension (PASP-Doppler), 3) coexisting diseases, 4) functional class of heart failure (NYHA). Most common abnormalities found were supraventricular tachyarrhythmias (22.8 pre- and 40.6% postoperatively). There was significant increase in frequency of arrhythmias after surgery in the subgroups II and III. Arrhythmias were most frequent in patients with coexisting arrhythmogenic diseases.