Spontaneous rupture of the extensor tendons of the hand has been found in five patients with long-standing rheumatoid arthritis and subluxation of the distal end of the ulna at the inferior radio-ulnar joint. Findings at operation in three of these patients indicated that the rupture was caused by degenerative changes in the floor of the extensortendon groove in the region of the distal end of the ulna. Satisfactory results were obtained by tendon repair combined with resection of the distal end of the ulna.