In this report we describe the clinical and pathological findings of cuff-tear arthropathy in twenty-six patients and discuss the differential diagnosis and a hypothesis on the pathomechanics that lead to its development. This lesion is thought to be peculiar to the glenohumeral joint because of the unique anatomy of the rotator cuff. Following a massive tear of the rotator cuff there is inactivity and disuse of the shoulder, leaking of the synovial fluid, and instability of the humeral head. These events in turn result in both nutritional and mechanical factors that cause atrophy of the glenohumeral articular cartilage and osteoporosis of the subchondral bone of the humeral head. A massive tear also allows the humeral head to be displaced upward, causing subacromial impingement that in time erodes the anterior portion of the acromion and the acromioclavicular joint. Eventually the soft, atrophic head collapses, producing the complete syndrome of cuff-tear arthropathy. The incongruous head may eventually erode the glenoid so deeply that the coracoid becomes eroded as well. Although treatment of cuff-tear arthropathy is extremely difficult, the preferred method appears to be a resurfacing total shoulder replacement with rotator-cuff reconstruction and special rehabilitation. We think that it is important to recognize cuff-tear arthropathy as a distinct pathological entity, as such recognition enhances our understanding of the more common impingement lesions. Cuff-tear arthropathy is especially difficult to treat, and although many tears of the rotator cuff do not enlarge sufficiently to allow this condition to develop, it is a factor to consider when deciding whether or not a documented tear of the rotator cuff should be surgically repaired.