Purpose: Renal dimensions are an important assessment of the genitourinary tract used to evaluate critical aspects of renal growth and development. Understanding the effect of patient position is important to use and interpret these parameters. In this prospective study we determined the effect of patient position and general body habitus on renal length and parenchymal area in children undergoing renal ultrasound. Materials and Methods: Between October 2010 and January 2011 children underwent renal ultrasound while prone and supine. Bilateral renal length and renal parenchymal area were measured. Pearson and Bland-Altman statistical analyses were used to examine correlations, measurement bias and the degree of agreement between methods. Results: Renal length measurements in both positions were complete for 201 right and 196 left kidneys. Parenchymal area measurements were complete for both kidneys in 177 children. When compared individually, supine and prone measures of renal length and parenchymal area highly correlated on Pearson analysis (greater than 0.96 and greater than 0.89, respectively). When compared by method, Bland-Altman analyses of differences vs means showed greater than 50% variance, representing wide limits of agreement with poor interrelation. Neither persistent systematic bias nor body habitus influenced results. Conclusions: While Pearson analysis showed high correlation for supine and prone renal measurements, Bland-Altman analysis of renal length and parenchymal area demonstrated wide limits of agreement, not allowing interchangeable use of prone and supine measurements. As such, renal ultrasound should specify standardized positions and benchmarks. These results provide guidance to standardize renal ultrasound measurements when renal size is used as an indicator of kidney health.