Collagen is the main supportive protein of connective tissue. Another of its functions is the initiation of haemostasis by activation of the platelets. It has been suggested that collagen is abnormal in idiopathic scoliosis. The present study lends further support to this view: collagen of fascia specimens from patients with adolescent idiopathic scoliosis was found to aggregate platelets less readily than collagen from non-scoliotic controls and suspensions of fascia patients with adolescent idiopathic scoliosis contained more collagen than those from the controls. The collagen abnormality is probably also reflected in a prolonged bleeding time. The changes in the collagen in patients with scoliosis persist at least for some years after the cessation of growth.