In 14 patients with an indwelling catheter, 48 different determinations of bladder volume were made using 5 different ultrasonic methods. The simplest procedure, the largest sagittal diameter of the bladder in the median plane, gave only a rough estimate of the bladder volume. The product of bladder depth, height, and width, as determined from transverse and sagittal scans, showed the best correlation to the true bladder volume (r=0.90). Computerized reconstruction of multiple serial cross sections proved less accurate than the much simpler depth, height, width product. None of the methods for quantitation of small volumes was good, but qualitative determination of small volumes was accurate.