For the study of contribution of atrial fibrillation (AF) during acute coronary syndrome (ACS) in long-term prognosis after clinical stabilization we examined 453 patients admitted to Moscow hospitals and followed them for 2.07 +/- 0.48 years. The following events were registered: fatal and nonfatal myocardial infarction (MI), unstable angina (UA), fatal and nonfatal stroke, death of other causes. At ACS onset sinus rhythm was noted in 419 (92.5%), permanent or persistent AF-in 16 (3.5%), attack (paroxysm) of AF - in 18 (4.0%) patients. Mean length of life before end point was 884.9 +/- 23.4, 827.3 +/- 123.3 and 514.0 +/- 111.3 days in patients with sinus rhythm, permanent/persistent AF, and attack of AF during first 10 days of ACS, respectively (p<0.001). Compared with patients in sinus rhythm in patients with attack of AF relative risk (RR) of occurrence of any end point was 1.75 (95% confidence interval [CI] 1.284 to 2.873, p< 0.001), of fatal MI - 1.72 (95% CI 1.026 to 2.873, p=0.040), of UA - 2.116 (95% CI 1.249 to 3.585, p=0.005), of stroke - 2.863 (95% CI 1.300 to 6.301, p=0.009). Multifactorial analysis selected history of MI and attack of AF during first 10 days of ACS as independent predictors of unfavorable outcome. Thus paroxysmal form of AF during hospital stay because of ACS is associated with higher probability of development of unfavorable events in the next 1-2 years.