Scapho-trapezoid osteoarthritis may accompany osteoarthritis of the CM joint of the thumb. If the scapho-trapezoid joint is not excised when performing an excision of the trapezium with ligament reconstruction and interposition of the tendon of the FCR, the procedure may be unsuccessful. We describe three patients with four thumbs in which scapho-trapezoid osteoarthritis was associated with basal joint arthritis. In one patient, it was not recognized pre-operatively and persistent post-operative pain dictated further surgery. In the other patients, the scapho-trapezoid joint was excised primarily, with resolution of the pain. In the pre-operative assessment, the scapho-trapezoid joint should be carefully studied to prevent failure of the procedure.