Seventy per cent of cases of osteoarthrosis of the hip are regarded as being 'idiopathic' in type. Idiopathic osteoarthrosis of the hip is twice as common in females as in males. Osteoarthrosis of the hands shows a definite association with idiopathic osteoarthrosis of the hip, especially in women. Osteoarthrosis of the knees is found in over half the cases of idiopathic osteoarthrosis of the hip and of the hands, although there are sex differences in the distribution of hand and knee osteoarthrosis in these cases. Hand osteoarthrosis is not related to hand laterality, although osteoarthritic changes are likely to be more severe in the dominant hand when there is osteoarthrosis of both hands. No convincing evidence was found of a relationship between the various slab radiograph patterns from femoral heads and idiopathic osteoarthrosis of the hip. There was no definite association between idiopathic osteoarthrosis and the presence of various tissue antigens. The identification of a group of patients with idiopathic osteoarthrosis suggests a generalised cartilage abnormality, with the development of osteoarthrosis being possible related to subsequent stress changes.