Extensor pollicis longus tendon rupture occurs in 0.2% to 3% of fractures of the distal radius. The underlying mechanism, or mechanisms, of rupture are unknown. This study evaluated this tendon and its surrounding structures using high-resolution ultrasound in 62 patients with distal radius fracture at 6 weeks after injury. The uninjured wrist was examined as a control. The ultrasound measurements at 6 weeks were then correlated with the patients' presenting X-rays. Significant findings included a reduced peak velocity of tendon movement and an increased thickness of both the extensor retinaculum and the tendon sheath on the fractured side. In respect of tendon sheath thickness, these changes were particularly associated with intraarticular fractures. We postulate that the increased thickness of the EPL tendon sheath may further impair an already tenuous blood supply and/or affect the diffusion of nutrients to the tendon within the third extensor compartment, leading to tendon attrition and rupture.