We studied a pair of 36-year-old identical twins with keratoconus, one of whom had undergone bilateral penetrating keratoplasties. Both patients had final best corrected Snellen visual acuity of approximately 20/20. However, contrast sensitivity functions for the unoperated twin were abnormal when compared to the operated twin. In addition, contrast sensitivity data for the operated twin during and after a graft rejection indicate that corneal graft opacity and distortion can significantly alter the contrast sensitivity function. The unusual occurrence of identical twins with keratoconus afforded the unique opportunity to explore the visual and psychophysical aspects of keratoconus in a genetically controlled setting.