Using a modified real-time phased array sector scanner, phase aberrations and amplitude fluctuations across the imaging aperture have been measured in a number of human subjects. Data from these subjects were classified into two categories based on the quality of conventional longitudinal images of the liver. Measured phase aberrations were very small in all subjects exhibiting high quality images. In contrast, large phase aberrations were measured in subjects producing low quality images. However, there were no significant amplitude variations across the array for all subjects studied. These results suggest that the absence of significant phase aberrations is a necessary condition for high quality phased array imaging. If so, improvements in clinical image quality in such subjects may be possible.