A series of incidence estimation studies of cancers among Koreans through a nationwide survey has been undertaken by authors since 1988. The medical records were studied of inpatients with diagnoses of either ICD-9 151 (malignant neoplasm of the stomach), or 197 (secondary malignant neoplasm of the respiratory and digestive systems), or 211 (benign neoplasm of other parts of the digestive system) in claims sent in by medical care institutions throughout the country to the Korea Medical Insurance Corporation (KMIC) during the period from January 1, 1986 to December 31, 1987. These records were abstracted in order to identify and confirm the new cases of stomach cancer among the beneficiaries of the KMIC, which covers about 10% of whole Korean population. Using these data from the KMIC, the incidence patterns of stomach cancer among Koreans were estimated as of July 1, 1986 to June 30, 1987. The crude incidence rates of stomach cancer among Koreans are estimated to be 36.2 (95% tonfidence interval; 35.3-36.9) and 21.0 (95% CI; 20.3-21.6) per 100,000 in males and females, respectively. The cumulative rates for age spans 0-64 and 0-74 are 3.8% and 7.3% in males, respectively. In females they are 1.8% and 3.0%. The adjusted rates for the world population are 57.9 in males and 25.1 in females, which are similar to those of Shanghai, China '78-'82 but lower than those of Osaka, Japan. The truncated rates for ages 35-64 years, however, are 108.3 in males and 49.1 in females, which may be the highest in the world. Among Koreans in Korea, an increased risk of stomach cancer in this age group is the notable finding. Incidence patterns of stomach cancer by age, sex, and area, which are the first report in Korea, are analyzed and presented.