The present study is in agreement with the Lyon hypothesis as indicated by the random pattern of vertical bands of normal-appearing and hypoplastic enamel in the proposita and the mother. Further studies which would quantitate the amount of normal-appearing and hypoplastic enamel in a lage number of teeth with the X-linked hypoplastic type of amelogenesis imperfecta may provide further data in support of the Lyon hypothesis as it applies to this condition. Even though the methodology employed in this study involves nothing more sophisticated than pedigree analysis and clinical appraisal, it purports to demonstrate and support the validity of the Lyon hypothesis concerning amelogenesis imperfecta in man.