It is conceivable that intramolecular and intermolecular crosslinks of collagen may be involved in the formation of hypertrophic scar, but little is known about the relationship between hypertrophic scar and crosslinks of collagen. We have isolated a new crosslinking amino acid from collagen and have named it pyridinoline. In this investigation, we examined the content of pyridinoline in human normal skin, mature scar and hypertrophic scar. An appreciable amount of pyridinoline was found in collagen of hypertrophic scar, but pyridinoline is virtually absent in collagen of normal skin.