Fragile X-linked mental retardation is a recently described entity that includes a chromosomal fragile site at Xq28 and macro-orchidism. We studied 50 institutionalized males and 15 noninstitutionalized males and found six (9.2%) with this disorder. Their clinical findings include enlarged testicular volumes, low IQs (30 to 40), perseverative speech patterns, and characteristic facial features, including prominent supraorbital ridges, prognathism, and large ears. We recommend cytogenetic studies for both males and females with mental retardation of unknown origin in order to establish early diagnoses and to extend proper genetic counseling to the affected families.