To determine the in vivo velocity of ultrasound (sound velocity) in the liver, an ultrasonic system which was composed of a pulsed Doppler flowmeter and electronic linear scanner was used. The principle of the velocity determination is as follows; If the sound velocity in the liver is larger than the "set up speed" of the system (1,530 m/sec, an average for human tissue), the sampling location of the flowmeter in a vessel at a right angle to the liver surface is outside the B-mode image of the vessel. Thus, by measuring the distance between a vessel and a sampling location, change of the sound velocity in the liver can be roughly detected. By using this system, sound velocity in the liver was roughly measured in 20 healthy adults and 30 patients with diffuse liver diseases. The results showed that the sound velocity was clearly increased in the cirrhotic liver and the velocity was strongly correlated to the ratio of stroma to total liver tissue (r=0.82). It is assumed that the measurement of the sound velocity in the liver is useful to characterize the morphological changes in diffuse liver diseases.