Medial displacement of the long tendon of the biceps brachii muscle is a rare condition associated with degenerative or traumatic ruptures of the rotator cuff. This condition was recorded in nine shoulders during 45 reconstructive procedures on the rotator cuff. Five of the displacements were complete, leaving the tendon medially displaced in a fascial sling; four were incomplete, allowing a to-and-fro medial displacement of the tendon out of the intertubercular groove. Anatomical dissections on eight shoulders showed that the coracohumeral ligament is the key ligament which keeps the biceps tendon aligned in the sulcus: transection of the medial part of the ligament allows the tendon to be medially displaced. There was no pathognomonic clinical sign of the luxation or subluxation. Since abnormal movement of the tendon in the sulcus may be an important cause of shoulder pain, the condition should be carefully looked for during reconstructive procedures on the rotator cuff. Tenodesis of the displaced tendon is recommended, either as the sole procedure or in combination with other reconstructive measures.