We ultrasonographically measured renal length in the major axis in children. Here we discuss the differences in measured lengths in the supine and prone positions. In study 1, which included 214 children, more than 60 percent of the kidneys measured longer in the supine than in the prone position. The difference (S-P: length in the supine position minus length in the prone position) was substantially greater in the left kidney. In study 2, we observed the relation between the kidney and the liver or spleen in 60 children prospectively to determine if artifacts had contributed to the observed differences. When the location of the liver or spleen was above the kidney, which may induce velocity displacement and renal duplication artifacts, the kidney tended to measure longer. We conclude that kidneys tend to measure longer in the supine than in the prone position. Difference in renal length in different positions was greater in the left kidney than in the right kidney. Sonographic artifacts are presumed to contribute to differences in measured length.