Obesity is associated with elevated blood pressure (BP) both in adults and children. Childhood obesity has become a severe health problem, especially during the last few decades. So far there has not been any large-scale study specifically focusing on the association between obesity and BP in early life. The aim of this study is to examine systematically the association between obesity and BP in preschool Chinese children in mainland China. In 1996, measurements of weight, height, and BP values were collected in a nationwide, case-control study of 748 boys and 574 girls who ranged in age from 0.1 to 6.9 years in 8 cities in mainland China, One obese child and 1 nonobese child were marched for Sender and age. The BP differences of the mean-matched pair were approximate to 5 mm Hg for systolic blood pressure (SBP) and approximate to 4 mm Hg for diastolic blood pressure (DBP) (P<0.05); a higher value was noted in obese children. The BP value of 19.4% children in the obese group and 7.0% children in the nonobese group was higher than the 95th percentile Value (P<0.0001), which is defined as high BP by the Task Force on Blood Pressure Control in Children. Both SBP and DBP were significantly (P<0.05) positively related to body mass index (BMI) values (P<0.05) for children in obese and nonobese groups after adjustment for age, gender, and height. To be specific, an increase of 1 BMI unit was associated with, on average, an increase of 0.56 mm Hg and 0.54 mm Hg in SBP and DBP, respectively, for obese children. In nonobese children, the increase in SBP and DBP was 1.22 mm Hg and 1.20 mm Hg, respectively. An increase in the BMI is conclusively associated with elevated SBP and DBP in nonobese children. Furthermore, an increase in the adjusted BMI was associated with an increase in SBP and DBP in obese and nonobese children.