To examine to which depth of the cornea the stiffening effect is biomechanically detectable. Department of Ophthalmology, University of Dresden, Dresden, Germany. Of 40 enucleated porcine eyes, 20 eyes were treated with the photosensitizer riboflavin (0.1%) and ultraviolet A (UVA) light (370 nm, 3 mW/cm2, 30 minutes); the other 20 eyes served as control. From each eye, 2 flaps of 200 microm thickness were cut with a microkeratome, and strips of 5 mm width and 7 mm length were prepared. Stress-strain behavior was measured with a material tester to characterize the stiffening effect. Five pairs of human donor eyes were tested in the same way. In porcine corneas, the stiffening effect was stronger in the anterior-treated flaps than in the posterior-treated flaps and the control flaps (P = .001). A 5% strain was achieved at a stress of 261.7 +/- 133.2 x 10(3) N/m2 in the anterior-treated flaps and 104.1 +/- 52.7 x 10(3) N/m2 in the anterior control flaps. The posterior-treated flaps (105.0 +/- 55.8 x 10(3) N/m2) and the posterior control flaps (103.7 +/- 61.8 x 10(3) N/m2) showed no difference (P = .95). A similar stiffening effect was observed in human eyes, but contrary to findings in porcine corneas, in human corneas the anterior control flaps were stiffer than the posterior control flaps (P = .027). Treatment of the cornea with riboflavin and UVA significantly stiffened the cornea only in the anterior 200 microm. This depth-dependent stiffening effect may be explained by the absorption behavior for UVA in the riboflavin-treated cornea. Sixty-five percent to 70% of UVA irradiation was absorbed within the anterior 200 microm and only 20% in the next 200 microm. Therefore, deeper structures and even the endothelium are not affected.