A septum between the abductor pollicis longus tendon and the extensor pollicis brevis tendon in the first extensor compartment is frequently encountered during the surgical treatment of de Quervain's disease. But de Quervain's disease associated with a bone tumor and an osseous septum is very rare. We treated a patient with de Quervain's disease who had a bone tumor and an osseous septum between the abductor pollicis longus tendon and the extensor pollicis brevis tendon in the first extensor compartment. After the excision, the pain was relieved and Finkelstein's test was negtive. We report upon this case, and includea brief review of the literature.