Objectives: To study a Kuwaiti family with fragile X mental retardation syndrome and present a rapid, noninvasive antibody test to be used for fragile X syndrome diagnosis on all mentally retarded patients in Kuwait. Methods: For cytogenetic analysis, the fragile site was induced by culturing blood lymphocytes in a medium containing a low concentration of folate (M199) or by adding 0.1 < mu >M of fluorodeoxyuridine to the culture mediFor the immunohistochemical test on blood smears, monoclonal antibodies against fragile X mental retardation (FMR1) gene product were used. The test is based on the presence of the protein in lymphocytes from normal individuals and its absence in lymphocytes from fragile X patients. Results: A Kuwaiti family with fragile X syndrome in 4 affected males is described. The immunohistochemical study with the monoclonal antibodies against FMR protein gave conclusive results on the same day for the affected males. Conclusion: To our knowledge, this is the first report describing a large Kuwaiti family with the fragile X syndrome. These data suggest that the immunohistochemical test used is an ideal method for the diagnosis of fragile X syndrome in mentally retarded patients in Kuwait.