A mathematical derivation of the effect of penetrating keratoplasty on corneal curvature was used to examine many variables in corneal surgery. The amount of wound disparity taken up by the recipient cornea was found to be the major factor in determining the amount of astigmatism induced by host wound/donor tissue size disparities. The amount of disparity showed as essentially linear relationship with the amount of astigmatism, approximately 0.4 diopters for each 0.1 mm of wound disparity for each 10% of the amount of the distortion taken up by the cornea (7.5 mm trephine). The smaller the trephine, the more distortion could be expected for each increment of wound disparity. Variations in the corneal curvature of the donor cornea had a minimal effect on the amount of keratoplasty-induced astigmatism.