Background. African Americans are at increased risk for hypertension and chronic renal disease. Some data suggest this results from renal microvascular disease. The aim of this study was to determine if renal vascular changes were more pronounced in African Americans, were independent of blood pressure, and occurred in early childhood Methods. We performed morphometric analysis on small cortical arteries and arterioles from 44 renal biopsies done in African American and white children (mean age 8.4 ± SD 5.0 years) with minimal change nephropathy. Outer and inner vessel diameters were measured and wall:lumen and wall:outer diameter ratios (WT/OD) calculated. Clinical data on blood pressure, steroid use, serum creatinine, gender, age, and proteinuria were abstracted by chart review. A z score for systolic and diastolic blood pressure was calculated. Follow-up clinical data were available for 11 children. Data were compared using analysis of covariance (ANCOVA) and t test for paired data. Results. Lumen diameters of African Americans were 3.1 μm (23%) smaller that those of white children (P = 0.024). Similarly, their WT/OD was greater than in the whites, 0.31±0.03 vs. 0.28 ± 0.02 (P = 0.048). These changes were independent of age, steroid use, systolic blood pressure and diastolic blood pressure z scores. Follow-up data showed a rise in serum creatinine (>50%) in five patients, +1.42 ± 0.79 mg/dL (P = 0.016), of whom four were African American. There was no change in blood pressure. Conclusion. The renal arterioles of African American children with minimal change nephropathy exhibit significantly smaller lumens and thicker walls than white children. The changes occur very early in life and are independent of age, blood pressure, and steroid use. Such changes may contribute to the African American predisposition to chronic renal disease and hypertension. © 2005 by the International Society of Nephrology.