BACKGROUND The cardiopulmonary stress test (CPST) allows the objective assessment of stress capacity in patients (pts) with heart failure (HF). There are a small number of studies concerning CPST in pts with HF and AF. AIM to assess the influence of AF on CPST parameters in pts with HF. MATERIAL The study group consisted of 56 males aged 35-65 years (x = 57.3 year) with diagnosed HF lasting at least 3 months, in the II and I NYHA functional class and with l eft ventricular ejection fraction(EF) below 40%. 12 pts had an idiopathic cardiomyopathy and 44--cardiomyopathy of ischemic origin. All study pts were divided into two groups: group I (gr. I) - with persistent AF and group II (gr. II)--with sinus rhythm. METHODS The BNP ad IL-6 levels were measured and transthoracic echocardiography and CPST were performed in all study pts.We have analysed the echocardiographic and CPST parameters in both study groups. RESULTS We found no significant differences in age, BMI, ethiology, left ventricular echocardiographic parameter values and NYHA functional class between both groups. In pts with AF (gr. I) we found significantly higher LA diameter (mm) 49.5, SD = 6.7 vs. 44.8, SD = 6.7 (p = 0.01), significantly higher BNP level (pg/ml) 582.75, SD = 179.36 vs. 442.94, SD = 213.75 (p = 0.03) and IL-6 level (pg/ml) 13.55, SD = 10.94 vs. 8.6, SD = 7.83 (p = 0.05). We also found significant differences in: HRmin (bpm) (gr. I--98.45, SD = 21.70, gr. II--80.48, SD = 15.25 p = 0.001), HRmax (bpm) (gr. I--146.82, SD = 16.14, gr. II--123,14, SD = 16.69 p = 0.001), tmax (sec) (gr. I--414.0, SD = 252.33, gr. II--618.11, SD=268,69, p = 0.02), METS (gr. I--2.09 SD = 1.04, gr. II--4.42, SD = 2.15 p = 0.002) and VO2peak (ml/kg/min) (gr. I--11.52, SD = 1.86, gr. II - 15,03, SD = 4.42, p = 0.01). CONCLUSIONS (1) Persistent atrial fibrillation in pts with HF is associated with 23% lower VO2peak and lower stress tolerance as compared to pts with sinus rhythm. (2) Higher IL-6 level in pts with HF and AF indicates the sympatethic system's overactivation and the presence of inflammatory reaction in these pts.