Etiology of class-III malocclusion is generally believed to be genetic. A wide range of environmental factors have been suggested as contributing factors for the development of class-III malocclusion. Twin study is one of the most effective methods available for investigating genetically determined variables of malocclusion. Discordancy for class-III malocclusion is a frequent finding in dizygotic twins. However, class-III malocclusion discordancy in monozygotic twins is a rare finding. The purpose of this study of monozygotic twins is to assess the genetic and environmental components of variation within the cranio-dento-facial complex.