The width of the symphysis pubis was measured in a skeleton model using slide rule, ultrasonography and x-ray. Ultrasonography gave a 0.5-mm and x-ray a 1.0-mm narrower gap than the slide rule. Further, 15 nonpregnant females undergoing urography volunteered for an extra ultrasonography of the symphysis pubis. The widths of the symphysis as measured from the x-ray exposures and the ultrasonography were compared. Regression analysis gave a regression coefficient of 0.979 and a correlation coefficient of 0.850. Ultrasonography offers at least the same precision as x-ray for assessing the width of the symphyseal gap.