The goal of this study is to better understand the trend in epidemiological features and the outcomes of emergency medical service (EMS)-assessed out-of-hospital cardiac arrest (OHCA) according to the community urbanization level: metropolitan, urban, and rural.This study was performed within a nationwide EMS system with a single-tiered basic-to-intermediate service level and approximately 900 destination hospitals for eligible OHCA cases in South Korea (with 48 million people). A nationwide OHCA database, which included information regarding demographics, Utstein criteria, EMS, and hospital factors and outcomes, was constructed using the EMS run sheets of eligible cases who were transported by 119 EMS ambulances and followed by a medical record review from 2006 to 2010. Cases with an unknown outcome were excluded. The community urbanization level was categorized according to population size, with metropolitan areas (more than 500,000 residents), urban areas (100,000-500,000 residents), and rural areas (<100,000 residents). The primary end point was the survival to discharge rate. Age- and sex-adjusted survival rates (ASRs) and standardized survival ratios (SSRs) with 95% confidence intervals (CIs) were calculated compared to a standard population. The adjusted odds ratios (AORs) and 95% CIs for survival were calculated and adjusted for potential risk factors using stratified multivariable logistic regression analysis.There were 97,291 EMS-assessed OHCAs with 73,826 (75.9%) EMS-treated cases analyzed, after excluding the patients with unknown outcome (N=4172). The standardized incidence rate increased from 37.5 in 2006 to 46.8 in 2010 per 100,000 person-years for EMS-assessed OHCAs, and the survival rate was 3.0% for EMS-assessed OHCAs (3.3% for cardiac etiology and 2.3% for non-cardiac etiology) and 3.6% for EMS-treated OHCAs. Significantly different trends were found by urbanization level for bystander CPR, EMS performance, and the level of the destination hospital. The ASRs for survival were significantly improved by year in the metropolitan areas (3.6% in 2006 to 5.3% in 2010) but remained low in the urban areas (1.4% in 2006 to 2.3% in 2010) and very low in the rural areas (0.5 in 2006 and 0.8 in 2010). The SSRs (95% CIs) in the metropolitan areas were 1.19 (1.06-1.34) in 2006 and 1.77 (1.64-1.92) in 2010, whereas the SSRs were observed to be less than 1.00 during the five-year period in both urban and rural areas. The AORs (95% CIs) for survival significantly increased to 1.42 (1.22-1.66) in the metropolitan areas and to 1.58 (1.18-2.11) in the urban areas while not increasing in the rural areas, compared to the level of each group of areas in 2006.In this nationwide cohort study from 2006 to 2010, the standardized incidence rate and survival to discharge rate of EMS-assessed OHCAs increased annually in metropolitan and urban communities but did not increase in rural communities. Further investigations should be undertaken to improve the performance and outcomes in rural communities.