Sick Sinus Syndrome describes the disorder of the sinus node function manifested by a wide range of heart rate disorders. In symptomatic patients, the only treatment is implantation of pacemaker. The AAI mode of pacing is recognized as the most physiological mode of pacing, but with the risk of the development of the AV block. The aim of the study:  estimation of incidence of AV block development in patients with Sick Sinus Syndrom,  The progression rate of the 1st AV block in the AV block of higher degree,  The incidence of Atrial fibrillation and ventricular frequency dependence of AV conduction. Subjects and Methods A study was performed on a sample of 70 patients with sick sinus syndrome. Due to the symptomatic sick sinus syndrome, the pacemaker was implanted in all the subjects. When pacemaker was implanted they had a normal AV conduction and were free of atrial fibrillation. All subjects are regularly controlled in the Ambulatory for arrhythmia and heart electrostimulation, Clinic for treatment, prevention and rehabilitation of heart and blood vessels, Special Hospital Thalassotherapia Opatija. This study is retrospective. The data are obtained from the documentation that contains all the relevant data at the time of pacemaker implantation and with regular check-ups performed twice or more times a year. The ECG finding was made during pacemaker implantation and under regular controls. The average time of follow-up was 8.6 years. Results The annual incidence of 1st degree AV block development is 0,66%, and 2nd and 3rd degree AV block is 0.1%. Progression from 1st degree AV block to higher degree AV blocks slow and rare. In average the higher degree AV block developed 29 months after the 1st degree AV appeared. The annual incidence of atrial fibrillation is 1.4%. The ventricular rate in atrial fibrillation is dependent on AV conduction. In patients with normal AV conduction the ventricular frequencies in atrial fibrillation are normal or higher. In the presence of the AV block, atrial fibrillation resultedin significant bradyarrhythmia of the ventricle. Conclusion The incidence of AV block development is low. The AAI mode of pacing in patients with sick sinus syndrome and normal AV conduction is a safe and optimal mode of pacing.In the occurrence of the AV block of stage 1, the AAI mode must be changed in DDD mode of pacing due to the possible progression of the AV block from the 1st degree to a higher degree or the onset of atrial fibrillation which will result in significant ventricular bradyarrhythmia.