Body mass has been consistently associated with blood pressure in acculturated populations but not in lean populations with low blood pressures. In southwestern China, in 1989, the authors studied the relation between body mass and blood pressure in three community-based random samples: rural Yi farmers (5,023 men, 3,218 women); Yi migrants (1,656 men, 919 women); and Han Chinese living in an urban setting (2,173 men, 1,516 women). The Yi farmers had virtually no obesity or hypertension and had little rise in blood pressure with age. The Yi migrants and Han had a steeper slope of blood pressure with age and consequently more hypertension. For men and women, respectively, mean body mass index (weight (kg)/height (m)2) was 20.6 and 21.0 in the Yi farmers, 21.3 and 21.4 in the Yi migrants, and 21.0 and 21.4 in the Han. Both systolic and diastolic blood pressure were positively related to body mass index in all six ethnicity-sex groups, and the association remained statistically significant after adjustment for age, heart rate, smoking, alcohol intake, and physical activity. The change in mean blood pressure for each kg/m2 increase in body mass index, after adjustment for community of residence, was 1.47 mmHg for systolic pressure and 1.13 mmHg for diastolic pressure. The association between body mass index and blood pressure was greater in men than in women and greater in Yi migrants and Han than in Yi farmers. The percentages of hypertension attributable to overweight (body mass index > or = 25) among the Yi farmers, Yi migrants, and Han, respectively, were 4.1%, 34.1%, and 24.0% for men and 0%, 26.2%, and 28.9% for women. Thus, even in this lean Chinese population with a low mean blood pressure, body mass was positively and independently associated with increased blood pressure.