A case of incomplete rupture of the extensor tendon to the little finger is reported in a 64-year-old woman with an idiopathic Madelung's deformity which had been present since adolescence but had given rise to no problems. Surgical treatment by resection of the head of the ulna with fixation of the ulna to the radius gave a satisfactory result. Only two similar case reports of rupture of extensor tendons associated with Madelung's deformity have been published. In spite of its rarity, this complication deserves to be known and possibly prevented by resection of the head of the ulna when it is very prominent. It should be recognized before any loss of extension of the little finger, however slight, develops. Lesions of extensor tendons in contact with an excessive prominence of the head of the ulna are among the best known complications of rheumatoid arthritis. Occasionally, the same type of lesion can be seen in Madelung's deformity.