Recent reports have proposed endoscopy as an alternative strategy to radiography for gastric cancer (GC) screening. The current study presents the first reported population-based data from a large GC screening program that provided endoscopic examinations. A retrospective population-based study was conducted using the National Cancer Screening Program (NCSP) database. We evaluated GC detection rates, sensitivity, specificity, and the positive predictive value of an endoscopic screening program for the average-risk Korean population, aged 40 years and older, who underwent the NCSP from 2002 to 2005. The detection rates of GC by endoscopy in the first and subsequent rounds were 2.71 and 2.14 per 1000 examinations, respectively. Localized cancer accounted for 45.7% of screen-detected GC cases. The sensitivity of endoscopy was 69% (95% confidence interval [CI]: 66.3-71.8). The endoscopic screening was less sensitive for the detection of localized GC (65.7%, 95% CI = 61.8-69.5) than for regional or distant GC (73.6%, 95% CI = 67.4-79.8). In the multiple logistic models for localized GC and all combined GC, the odds ratio (OR) of sensitivity for the undifferentiated type was statistically significantly higher than that for the differentiated type, whereas the OR of sensitivity for the mixed type was lower than that for the differentiated type. The sensitivity of the endoscopic test in a population-based screening was slightly higher for the detection of regional or distant GC than for localized GC. Further evaluation of the impact of endoscopic screening should take into account the balance of cost and mortality reduction. (Cancer Sci 2011; 102: 1559-1564 G astric cancer (GC) is the fourth most common type of cancer (934 000 new cases, 8.6% of all new cancer cases in 2002), and the second most common cause of cancer death (700 000 deaths annually) in the world.(1) Several Asian countries, including China, Japan, and Korea, have the highest incidences of GC in the world.(1) Although the incidence of GC in Korea has declined in recent decades, it remains the most common cancer. (2) Because the prognosis of early GC is highly favorable, highprevalence countries, such as Japan and Korea, have sought to reduce the disease burden by providing GC screening to average-risk populations. Since 1960, Japan has conducted mass screening for GC with photofluorography (via indirect upper gastrointestinal series), achieving remarkable improvement in survival rates as a result of early detection, and consequently, higher cure rates. (3)(4)(5) This use of photofluorography for GC screening in Japan is based on the results of several case-control and cohort studies.(6) Although population-based photofluorography screening has been mandated as a public policy matter, other opportunistic screening methods have been used in the clinical setting, including endoscopy, serum pepsinogen testing, and Helicobacter pylori antibody testing.(6) However, the efficacy of these methods remains unclear. In Korea, national GC screening was instituted i...