The "comet tail" is a well-known ultrasound artifact that appears as a series of parallel bands radiating from a source. This artifact was observed in the gallbladder lumen in eight of 10 patients after extracorporeal shock wave lithotripsy of radiolucent calculi. To investigate the cause of the comet tail artifact, cholesterol crystals were studied in an in vitro model. The comet-tail appearance was reproduced in vitro and was found to be directly related to the interrogating frequency and the size of the crystal aggregates. The authors conclude that cholesterol crystals can serve as the source of the comet-tail artifact secondary to reverberation within the crystals.