Obstructive sleep apnea syndrome (OSAS) is associated with increased risk of different cardiovascular complications such as arterial hypertension, ischemic heart disease, rhythm disorders. Pericardial effusion (PE) is common condition in patients with OSAS, but usually in correlation with respiratory failure and pulmonary arterial hypertension (PAH). The aim of the study is to evaluate the presence of PE in OSAS patients without PAH and/or daily hypoxemia, and to assess the possible factors associated with it. 279 consecutive patients (162 men and 117 women, mean age 42.76 ± 12.44 years) with newly diagnosed OSAS were included. Different laboratory tests, 2-D echocardiography with Doppler-sonography to exclude other diseases that can lead to chronic PE were provided in all patients. PE was found in 102 (36.56%) of the patients.The mean effusion volume was small (150-250 ml). The PE significantly correlated with severity of OSA (r = 0.374, p<0.001), body mass index (r=0.473, p<0.001) and duration of sleep desaturation (r = 0.289, p<0.001). Persistent pericardial effusion is common in patients with OSAS. Although it is usually not life-threatening condition, the PE could be very resistant to treatment. The exact mechanisms for the development of PE in many patients with OSAS are still unclear but the correlation with obesity and desaturation time during sleep show that PE may be as a result of chronic low-grade inflammation and elevated systemic oxidative stress.