This case report describes a patient with an acromio-clavicular joint (ACJ) cyst, associated with a complete tear of the supraspinatus tendon, and the related arthropathy. Ultrasound was a suitable imaging modality to make the diagnosis, and rule out other pathologies. Full assessment of the rotator cuff must be carried out in the presence of ACJ cysts due to their common co-existence with large cuff tears. Cyst aspiration is not a suitable treatment, due to the high likelihood of recurrence. Optimal treatment requires management of the underlying rotator cuff tear. (C) 2013 Elsevier Ltd. All rights reserved.