BACKGROUND Permanent cardiac pacing is applied in symptomatic cases of sick sinus syndrome (SSS). The aim of our research was to characterize a group of patients suffering from SSS with low risk of developing into the first degree atrioventricular block (AVB), who are able to have the atrial pacemaker (AAI) implanted. METHODS 243 patients (mean aged 61.8 years, 22.7-90.3) with implanted AAI pacemaker due to SSS were qualified. The PQ interval was compared in the earliest found (1-119 months from implantation, SD +/- 2.7 m) and the last control examination of each patient. Mean follow-up time was 88,8 months. We took into consideration the age at the moment of implantation, basic rhythm rate, polarity of the stimulating electrode (unipolar/bipolar), and presence or absence of "rate response" option. RESULTS During the follow-up the PQ interval prolonged in 40.4% of patients, on average by 36 ms, did not change in 42.1%, and in 17.5% contracted, on average by 30 ms. The age differences of these groups were not statistically significant. Prolongation of PQ in atrial pacing was rather sudden than linear, below the certain age changes of PQ are slight and after going beyond this age it increases visibly. In our patients this crucial age is 60-70. Presence of "rate response", polarity of the stimulating electrode, basic rhythm rate did not have any influence on PQ changes. The 1st degree AVB has been developed almost in a half of our group of patients. They had longer, almost border PQ in the first examination (mean 196 ms, SD +/- 29 vs. 166 ms, SD +/- 24; p < 0.001). During on average 63 months long observation the PQ interval became longer on average by next 23 ms. In the rest of patients, during the comparable time period PQ did not change. The differences of PQ changes between these groups were strongly statistically significant (p < 0.001). The 1st degree AVB occurred in 20% of patients with initial PQ < or = 160 ms and in 53% of patients with longer PQ. CONCLUSIONS 1. The 1st degree AVB in the group of patients in 7th-8th decade of their lives, regardless of patients' sex, treated with permanent AAI pacing, develops usually when PQ reaches the upper borderline of standard range in the examination at the starting point. 2. In people with initially short PQ interval (< or = 160 ms) there is low risk of the 1st degree AVB.