The accuracy of ultrasonography in assessing the bladder volume in 13 children with normal bladder contours was determined using the formula D x H x W (depth x height x width) with a mean error of 15.7%. This formula tends to overestimate the bladder volume in these patients. Multiplication by a correction factor of 0.9 yields a mean error of 11.5%. The accuracy of this evaluation is not dependent upon the bladder volume as the mean error was the same in those children whose bladder volume was either less than or greater than 150 cc. In addition, the accuracy of ultrasonography in assessing bladder volume was evaluated in 7 patients with an abnormal bladder contour (trabeculation, diverticula, etc.); mean error was 14.7%. In these 7 patients there was no predictable under or overestimation of bladder volume. In the children with abnormal bladder contours on ultrasound, the correction factor of 0.9 should not be used, as it does not add to the accuracy of the procedure in any individual patient. This technique is still valuable, however, in children with an abnormal bladder for it allows estimation of the extent of emptying without invasive catheterization.