Rural-urban migration provides an ideal opportunity to examine the effects of environment and genes on blood pressure. The effect of migration on the Yi people of China was studied. The Yi people live in a remote mountain area in southwestern China. In 1989, blood pressure was measured in 14,505 persons (8,241 Yi farmers, 2,575 urban Yi migrants, and 3,689 Han urban residents) aged 15-89 years. Different patterns were seen for men and women. Among the men, Yi farmers had the lowest mean blood pressure, the least rise in blood pressure with age (systolic blood pressure, 0.13 mmHg/year; diastolic blood pressure, 0.23 mmHg/year), and the lowest prevalence of hypertension (0.66%). In contrast, both Yi migrant men and Han men had higher levels of mean blood pressure, rise in blood pressure with age (Yi migrants: systolic pressure, 0.33 mmHg/year; diastolic pressure, 0.33 mmHg/year; Han: systolic pressure, 0.36 mmHg/year; diastolic pressure, 0.23 mmHg/year), and prevalence of hypertension (Yi migrants, 4.25%; Han, 4.91%). Among the women, however, mean systolic pressure was higher in Yi farmers than in Yi migrants or in Han. Diastolic pressure was similar among the three groups. However, the Yi farmer women's age-related rise in blood pressure (systolic pressure, 0.06 mmHg/year; diastolic pressure, 0.14 mmHg/year) and their prevalence of hypertension (0.33%) were lower than those in the other two groups. Yi migrant women had an intermediate rise in blood pressure with age (systolic pressure, 0.37 mmHg/year; diastolic pressure, 0.23 mmHg/year) and prevalence of hypertension (2.40%). Han women had the greatest rise in blood pressure with age (systolic pressure, 0.56 mmHg/year; diastolic pressure, 0.36 mmHg/year) and the highest prevalence of hypertension (4.76%). For both men and women, the above differences were only partially explained by age, body mass index, heart rate, smoking, and alcohol use. This study, using standardized methods, demonstrates an important effect of migration on rise in blood pressure with age and on the prevalence of hypertension.