The most common heritable form of mental retardation is the fragile X syndrome. It is X-linked and affects 1:1500-1:4000 boys. In Denmark 230 affected individuals are known, thereby rendering it underdiagnosed. Only minor dysmorphic traits are associated with the syndrome, more pronounced in boys, namely a long face with large, prominent ears, and macroorchidism postpubertally. The psychological manifestations are autistic features, hyperactivity and deviant behavior. Today no medical curative treatment is available but much can be gained from social and educational intervention. The syndrome is caused by a dynamic mutation on the X chromosome and displays a remarkable pattern of inheritance. Carrier diagnosis and prenatal diagnosis are feasible. This article describes the clinical, diagnostic and genetic aspects of fragile X syndrome.