Asian immigrants to the U.S. are participants in a natural experiment on the effects of social norms on tobacco cessation. Smoking is socially acceptable in most Asian countries. When Asian smokers move to U.S. states such as California, they experience a radically different social norm toward smoking. This study examines ever smokers among two groups of Asian immigrants in California, Chinese and Koreans, and finds that most have quit smoking. The quit ratios (percent of ever smokers who have quit) for Chinese (52.5%) and Korean immigrants (51.1%) have quit ratios for ever smokers in California in general (53.3%), which is among the highest in the U.S. These high quit ratios contrast sharply with much lower quit ratios for Chinese in China (11.5%) and for Koreans in Korea (22.3%). Such large differences in quit ratios are the results of accumulated differences over the years, because of dramatic differences in annual cessation rates: Chinese in California quit at roughly seven times the rate of Chinese in China, and Koreans in California three times that of Koreans in Korea. Analyses further show that these large differences in annual cessation rates come mainly from the fact that these immigrants in California made quit attempts at a much higher rate than their counterparts in their home countries. These results suggest that creating an impetus to drive up quit attempts, which often results from a significant change in social norms toward smoking, is the most important strategy to improve cessation on the population level.