To determine migration-related differences in serum lipid levels and to estimate the contribution of environmental factors to these differences, the authors conducted a cross-sectional study among 1,222 residents of southwestern China in 1988 and 1989: 521 Yi farmers living in a remote rural area, 340 Yi migrants to an urban setting, and 361 Han people living in the same urban setting. Serum total and high density lipoprotein cholesterol and serum triglyceride were measured after a 14-hour fast. Information on age, cigarette smoking, alcohol consumption, and physical activity was obtained by questionnaire. Diet was assessed by use of three 24-hour diet recalls. The Yi farmers had the lowest age-adjusted serum lipid levels among the three groups, white the Yi migrants and their Han counterparts had similar lipid levels. After adjustment for age, body mass index, cigarette smoking, alcohol drinking, and dietary nutrient intake, the difference in serum lipid profile between the Yi migrants and the Yi farmers was substantially reduced. These results indicate that rural-urban migration is associated with higher serum lipid levels. This migration-related difference in serum lipids can be partially explained by changes in lifestyle and dietary nutrient intake. The authors conclude that environmental factors are important determinants of population serum lipid levels in southwestern Chinese.