The amount of lysine in collagen is only 3 or 4% of total aminoacids, but it has an important function in the constitution of the cross-links between the molecules to built the fibrils and the fibers of collagen. For this function, some lysine molecules must be hydroxylated and other oxidized to aldehyde compounds. Some deficits in these metabolic pathways are responsible for heritable diseases of the connectivite tissue as types IV and IX of the Ehlers-Danlos syndrome, lathyrism, the Menkes kinky hair syndrome, Cutis-Laxa or the type II of osteogenesis imperfecta. The contribution to fibrosis is also discussed.