A septum is not unusual within the first dorsal compartment of the wrist. The right and left forearms of 50 cadavers were dissected. A septum was identified 34% of the time. The septum separated the abductor pollicis longus tendon from the extensor pollicis brevis tendon. If the septum was present on one side, it was usually present on the other side. The septum created a separate compartment for the extensor pollicis brevis tendon. The effect of a separate compartment on the operative and nonoperative treatment of de Quervain's disease is discussed.