Changes in the relative frequency of gastric adenocarcinoma in southern California. The incidence of gastric cancer is decreasing in most counties of the developed world, but at the Los Angeles County-University of Southern California Medical Center, we diagnosed 99.8 cases of gastric adenocarcinoma per 10(5) discharges in the period 1982 to 1986 as opposed to 62.2 per 10(5) discharges in 1972 to 1976 (P less than .0001). This change involved primarily Hispanics younger than 30 years of age with 30 cases per 10(5) vs 4.2 cases per 10(5) (P less than .0001) and whites older than 30 years: 87 cases per 10(5) vs 54 cases per 10(5) (P less than .05) during 1982 to 1986 and 1972 to 1976, respectively. There was no change in the relative frequency rates of gastric adenocarcinoma among African Americans and Asians. Although these changes do not seem important enough to make the detection of gastric cancer a high-priority public health problem, they should alert physicians working in areas with high Hispanic populations of the relative possibility of the occurrence of gastric malignancy even in young patients. Also, we have found that gastric cancer is still prevalent in whites of low socioeconomic class. 