To study the impact of income change--specifically rapid income growth--on diet behavior over time and by socioeconomic level, we used data from a prospective study of China begun in 1989 (followed up in 1991, 1993 and 1997). The subpopulation used in this study included 5783 subjects aged 20-45 years old from 3129 households. Dietary intakes were measured using a combination of the weighing method and three consecutive 24-h recalls. Detailed income and price data were collected, and predicted household per capita income was used in multivariate longitudinal random-effects models that described the consumption of several food groups and nutrients. Income elasticity was used to measure the changes for the effects of income over time on (a) the probability of consuming any food and (b) the quantity of food consumed. The structure of the Chinese diet is shifting away from high-carbohydrate foods toward high-fat, high-energy density foods. The variation in the income effects that we uncovered indicated that important changes in income effects took place between 1989 and 1997, with the changes varying considerably by socioeconomic status. These shifts in income effects indicate that increased income might have affected diets and body composition in a detrimental manner to health, with those in low-income groups having the largest increase in detrimental effects due to increased income. Extrapolating from our estimates, higher income levels in the future could lead to the reversal of the health improvements achieved in the last two decades, if diet-related noncommunicable diseases cannot be controlled.