Stomach cancer is still the most common cancer in both males and females in Japan and the Japanese still show the highest mortality and incidence of stomach cancer in the world. However, the age-adjusted death rate of stomach cancer has shown a marked declining trend in both sexes for the last 25 years in Japan. Advocates of mass screening for stomach cancer and clinicians who specialize in the surgical treatment of stomach cancer may wish to claim credit for the decrease of stomach cancer mortality in Japan whereas advocates of primary prevention of cancer may wish to claim that the decrease of stomach cancer mortality is largely due to a decrease of stomach cancer incidence reflecting a recent change of dietary habits, especially the spread of western-style foods and diminished intake of traditional Japanese foods. In fact, the incidence rate of stomach cancer as estimated from the Osaka Cancer Registry shows a similar decreasing trend to the mortality. In many western countries, notably in the United States, the stomach cancer death rate has been decreasing for a long time, and is still decreasing. In those countries stomach cancer is now ranked as one of the rare cancers. It is hoped that the stomach cancer mortality and incidence will further decrease in the future in Japan in a similar way. Meanwhile, it is necessary to evaluate accurately the effectiveness of stomach cancer screening programs. It is also necessary and useful to explore the reasons for the recent decrease of stomach cancer incidence in Japan, which could be regarded as a "natural experiment" or "passive primary prevention." If we could identify the main reasons for the decrease of stomach cancer incidence from epidemiologic studies, it might be possible to speed-up the decrease of stomach cancer incidence in Japan, as well as in other countries which still show a relatively high incidence of stomach cancer.