The relative importance of the fat and muscle layers of the human abdominal wall in producing ultrasonic wavefront distortion has been assessed based on direct measurements. Specimens employed included 6 whole abdominal wall specimens and 12 partial specimens obtained by dividing each whole specimen into a fat and a muscle layer. In the measurements, a hemispheric transducer transmitted a 3.75-MHz ultrasonic pulse through a tissue section maintained at 37(degrees). The received wavefront was measured by a linear array translated in the elevation direction to form a two-dimensional aperture with overall dimensions 92.16x46.08 mm[sup 2] and a measurement spot size of 0.72x1.44 mm[sup 2]. Differences in arrival time and energy level between the measured waveforms and computed references that account for geometric delay and spreading were calculated. After correction for the effrects of geometry, the received waveforms were synthetically focused at 180 mm. The characteristics of the distortion produced by each specimen and the quality of the resulting foci were analyzed and compared. The results indicate that both fat and muscle layers contribute significantly to the distortion of ultrasonic beams by the abdominal wall. However, the spatial characteristics of the distortion produced by fat and muscle layers differ substantially, and the total distortion produced by the abdominal wall is not equivalent to the sum of the distortion produced by the layers.