During extracorporeal shock wave lithotripsy, changes in tissue echogenicity are observed by ultrasound. Their significance is not known. An experiment was performed in which 3,000 extracorporeal shock waves were applied under sonographic observation to the gallbladder wall of 6 dogs. No stones had been implanted, but transient shadows appeared simulating jumping stone fragments in the bladder. Echoes within the bladder lumen occurred in 3 dogs and were associated with hemorrhage into the lumen; in an additional ex vivo experiment, shock waves generated echoes in bile only after injection of a small amount of blood. In the liver, a transient increase in echogenicity was noted after a few shocks; it coincided with the region of tissue damage. Intense focal echoes occurred in the liver of 2 dogs at sites where a hematoma was found at autopsy. It is concluded that an increased focal echogenicity is an indicator of tissue damage by shock waves. The sonographic changes are thought to be caused by the transient generation of gas bubbles. The interaction of shock waves with gas bubbles is an established powerful mechanism which could explain the generation of tissue damage by shock waves.