In contrast to routine flexor tendon injuries, flexor tendon ruptures following blunt injury or re-ruptures following repair can be difficult to diagnose. The authors investigated the efficacy of using ultrasound to assist in the diagnosis. From 1996 to 1997, 8 patients underwent evaluation of the flexor tendons using an ATL HDI-3000 ultrasound machine with a high-resolution, 5 to 9-MHz hockey stick linear probe. Dynamic evaluation was performed in real time, simulating clinical symptoms. Six patients underwent surgical exploration. Sonographic diagnosis and intraoperative findings were correlated. Ultrasound was used to diagnose 3 patients with ruptured flexor digitorum profundus tendons. Mechanisms of injury included forceful extension, penetrating injury, and delayed rupture 3 weeks after tendon repair. Subsequent surgical exploration confirmed the ruptures and location of the stumps. Five patients had intact flexor tendons by ultrasound after forceful extension, penetrating injury, phalangeal fracture, crush injury, and unknown etiology. In 3 patients who underwent surgery for tenolysis, scar release, or arthrodesis, the flexor tendons were found to be intact, as predicted by ultrasound. The authors found ultrasound to be accurate in diagnosing the integrity of flexor tendons and in localizing the ruptured ends. They conclude that ultrasound is helpful in evaluating equivocal flexor tendon injuries.