A 23-year-old pregnant woman was referred to the Genetics Department for genetic counselling and possible prenatal diagnosis because of unexplained mental retardation in a paternal uncle. Family history revealed that her father had two mentally retarded sisters well as a brother with mental retardation and a normal sister. This normal sister had two normal sons, on normal daughter and one mentally retarded daughter. The normal daughter had one daughter, who has been referred to the pediatric department because of mental retardation two months earlier; Rett syndrome was diagnosed as this girl had the classical symptoms of this syndrome. The paternal uncle of the consultant was examined. H head typical features of fragile X syndrome. Therefore cytogenetic and molecular studies were undertaken in this family. Fragile X syndrome was diagnosed in the girl with Rett syndrome by the presence of the classical FMR-1 mutation and confirmed by cytogenetic studies, which revealed 12% fragile X positive cells.