We report a rare case of the locking of the extensor pollicis longus (EPL) tendon after fracture of the distal end of the radius. A 16-year-old boy was injured with comminuted-Smith type fracture in a traffic accident. After osteosynthesis, we found extension lag of the thumb. Seven days later, extension was impossible. Subcutaneous rupture of EPL tendon was suspected, but the tendon was caught in the fracture, and covered with callus and scar tissue. It was treated with tenolysis, and the extension lag disappeared in five weeks. It is important to find such cases in their early stage.
