No large national studies of ethnic differences in blood pressure among children accounting for body size differences have been published, to the authors' knowledge. This report details the similarities and differences in systolic and diastolic blood pressures between Black children and White children in the United States and examines the effects of age, sex, and body size on ethnic differences in blood pressure levels. Standardized measurements of seated systolic and diastolic pressures from eight large epidemiologic studies published between 1978 and 1991 that included measurements of 47,196 children on 68,556 occasions for systolic pressure and for 38,184 children on 52,053 occasions for diastolic pressure were used; 51 percent (24,048 children) were boys and 37 percent (17,466 children) were Black. Overall, there appear to be few substantive ethnic differences in either systolic or diastolic pressure during childhood and adolescence. The differences that were observed were small, inconsistent, and often explained by differences in body size. There was an ethnic group-body mass index (BMI) interaction that resulted in these findings that at lower levels of BMI Blacks have higher blood pressure and more hypertension than do Whites, but that at the highest levels of BMI, Whites have more hypertension (systolic or diastolic pressure) than do Blacks.