Keratoconus corneas are characterized by a reduced mechanical stability. The mechanical strength depends primarily on the collagen. In the present study the cross-linking pattern is analysed because it is important for the mechanical properties; an analysis of the amino acids of the collagen is included. Surprisingly, we did not find any change in the cross-linking pattern of collagen from keratoconus corneas compared to normal corneas. Neither did we find any difference in the composition of amino acids of the collagens. Thus no alteration was found in the molecular structure which could explain the marked reduction in the mechanical stability of keratoconus corneas.