We present two pairs of monozygotic twins discordant for keratoconus. Two pairs of twins, each with one twin with keratoconus, and available family members were examined clinically and with computer-assisted videokeratography. Polymerase chain reaction-based zygosity assays using between nine and 11 unique, anonymous DNA markers were performed on blood obtained from the twins and surviving parents to assess the probability of genetic monozygosity. DNA probes showed a >99% probability that each of the two sets of twins was monozygotic. One twin from each pair had clinically diagnosed keratoconus. The remaining twins were normal by clinical examination and corneal topography. Clinical results for all family members examined were normal except that five of 13 from one family and one of six from the other family demonstrated “suspicious” corneal topography. Recent advances in knowledge and understanding of the twinning process suggest that monozygotic twins discordant for keratoconus does not preclude the possibility of a significant genetic component.