Background: The aim of our study was to investigate the reliability of automated oscillometric blood pressure (BP) monitoring in the presence and absence of atrial fibrillation (AF) in hypertensive patients. Methods: BP was measured and compared in 71 randomly selected patients with AF and arterial hypertension diagnosis, 4 times each by auscultatory and oscillometric (Microlife BP A6 PC with AF detection system) methods. Results: Study included 71 patients: 36 males (mean age 67.4 years) and 35 females (70.2 years). At the time of BP measuring procedure, 36 patients were in sinus rhythm (SR) and 35 in AF. In SR patients mean systolic blood pressure (SBP) was 132 +/- 17.9 mmHg with auscultatory method (AM), 137.4 +/- 19.4 mmHg with oscillometric method (OM); mean diastolic BP was 77.1 +/- 10.9 mmHg (AM), 78.5 +/- 12.2 mmHg (OM), in AF patients mean SBP was 127.5 +/- 15.1 mmHg (AM), 133.6 +/- 17.4 mmHg (OM); mean diastolic BP was 81.4 +/- 9.9 mmHg (AM), 83.5 +/- 11.8 mmHg (OM), p = 0.037. The averages of differences for SBP and DBP in sinus rhythm group were (-5.3 mmHg (95% limits of agreement -27.2 -16.6)) and (-1. 4 mmHg (95% limits of agreement -12.8 -10.0)), respectively. In patients with AF the averages of differences for SBP and DBP were (-6.1 mmHg (95% limits of agreement -23.9 -11.7)) and (-2.1 mmHg (95% limits of agreement -12.9 -8.7)), respectively. Conclusions: The oscillometric device validated for patients with AF on average gives 5.3 mmHg higher systolic BP values for patients with SR and 6.3 mmHg higher BP values for patients with AF. However, the limits of agreement between two methods reveal wide range of random error rates which is a questionable topic in clinical practice, as it could possibly affect the treatment of arterial hypertension in patients with AF.