Although joint instability frequently follows certain types of polyarthritis it has seldom been investigated in the past; the end results vary from the nuisance of the unstable wrist to the catastrophe of atlanto-axial dislocation. It is upon the ligaments that stability so often depends, yet few studies have been performed to assess their functional state and physical characteristics either in health or disease. The aim of this investigation has been to provide some data to clarify the situation. Apart from the condition included in this study, joint instability is a feature of several diverse states. There are certain hereditary disorders of connective tissue in which there have been consistent reports of excessive joint mobility. Carter and Sweetnam (1958) described a family with very mobile joints complicated by recurrent dislocation of the patella -a condition recognized as familial joint laxity quite distinct from seemingly related conditions. McKusick (1960) and many others have reported similar mobile joints in osteogenesis imperfecta, Marfan's syndrome, and Ehlers-Danlos syndrome, in addition to the other manifestations. In all of these conditions there is some primary abnormality of the connective tissue. There are other conditions that produce instability because of their effect upon previously normal ligaments. Trauma is an important cause of this, and there have been numerous clinical reports, a good example of which is that of Ritchey (1960) who studied 28 cases of unstable knees following injury. Infection can produce an unstable jointapart from gross destructive change-in some unknown way. After a naso-pharyngeal infection, the transverse ligament of the atlas can give way leading to atlanto-axial dislocation. This complication has been recorded by Bell (1830), Grisel (1930), and Watson-Jones (1932), but the ligaments were not examined histologically, nor were any attempts made to measure strength or elasticity, so that the mechanism of the dislocation remains obscure. Similar dislocations have been reported