In four families with mental retardation as cardinal sign, authors found 11 cases of fragile chromosome X, 8 males (most with severe mental deficiency), and 3 females (mothers of affected children). Association of behavior perturbation as uneasiness, and hyperkinesia; facial dysmorphism as long or triangular face, big ears and macroorchidism, postpubertal in most of the cases, allow for clinical diagnosis. Autistic behavior is a suspicion sign too. Authors point out the necessity of a precocious diagnosis in order to treat patients with folic acid and to extend genetic counseling to the families. Prenatal identification is possible and inheritance mode in uncertain, debating recessive form linked to chromosome X and autosomic dominant controlled by sex. Incidence of 1.8/1,000 in newborn males, convert this specific form as the most frequent cause of mental retardation in this sex.