The ultrasound appearances of the neonatal and adult kidney contrast dramatically. In the normal kidney of a newborn infant, the echogenicity of the renal cortex equals that of the liver. To determine the morphological reasons for the unique and specific sonographic findings, we performed a quantitative anatomic analysis of the components of the kidney on autopsy specimens from four neonates. Our analysis reveals that the glomeruli occupy proportionally a much greater volume of the renal cortex during the first two months of life (18% as compared with 8.6% in the adult). Other anatomic differences include the greater proportional volume of the cellular component of the glomerular tuft in the neonate, and the location of 20% of the loops of Henle within the neonatal cortex rather than in the medulla. The increased number of anatomical structures, and thus interfaces, undoubtedly accounts for the increased echogenicity. The neonatal kidney often demonstrates prominent and hypoechoic renal pyramids. This may be explained by the larger volume of medulla in the neonatal kidney, which results in a ratio of cortex to medulla of 1.64:1 in the neonate as compared with a ratio of 2.59:1 in the adult. There is also a noted lack of echogenicity in the renal sinus region in neonates. Anatomically this is secondary to the absence of renal sinus adipose tissue in the kidney of the neonate.