- Background The past decade has brought major advances in therapy of patients presenting with acute coronary syndromes (ACS). Hypothesis The outcomes of ACS patients with atrial fibrillation (AF) has improved over the years. Methods We evaluated time-dependent changes in clinical characteristics, management strategies, and outcomes associated with AF observed during ACS hospitalization in patients enrolled in the biannual ACS Israeli Surveys (ACSIS) between 2000 and 2013. We divided the surveys into early (2000–2006) and late (2008–2013) time periods. Results The study population comprised 13 297 ACS patients, of whom 755 (5.7%) either presented with or developed AF during the index hospitalization. The incidence of AF observed during ACS hospitalization was lower during the later period (6.2% vs 4.9%; P < 0.01). Kaplan-Meier survival analysis showed that 1-year mortality rates were significantly lower among ACS patients with and without AF who were enrolled during the later vs early survey period (log-rank P < 0.01 for the overall difference). However, AF observed among ACS patients was still associated with increased risk for in-hospital complications and 30-day and 1-year mortality during both the early and late survey periods (P < 0.05 for all comparisons). Conclusions In 10 years, rates of AF among ACS patients have declined significantly. Yet, despite improved overall 1-year survival for patients with AF, the development of AF in this population remains an independent predictor of mortality that has not changed appreciably over the years.