Though Fragile X syndrome is one of the most common inherited causes of mental retardation, it is not much detected yet in Korean population. One of the reason may be that the syndrome is not well known to the special education teachers as well as to the clinicians in this country. Thus, molecular test was undertaken to screen out fragile X syndrome in 122 children of two Korean schools for emotionally severely handicapped children. The subjects were all boys, previously known as having pervasive developmental disorder with or without mental retardation. Southern blot analysis of peripheral blood showed the abnormally enlarged (CGG)n repeat sequence associated with fragile X syndrome in two children. This finding suggests that the DNA testing for fragile X syndrome is warranted for Korean high risk population and that more concern about this syndrome is needed for the professionals who work for mentally handicapped children. The issues involved in genetic counseling for fragile X syndrome are discussed.