Fragile X (Fra-X) Syndrome is the firt cause of heredity mental retardation(MR). Its incidence is 1/1000 males, and its inheritance is a singular form of an X-linked genic transmission. This syndrome has clinic features which allow us to suspect it. In 1989 we prepared a 16 pediatric clinical feature score, giving 1 point to each item (Familiar history. Height and cephallic circumference >p50, Long ears, Promunent ears, Long face, High palate, Hand's callus, Hyperlaxity, Depressed sternum, Macroorchidism, Poor visual contact, Hyperactivity, Motor stereotypies, Self agression, MR and Language disorders (SLAIP 1991). To evaluate this score we took a sample of 225 males with MR and/or Language disorders, without diagnosis, attending the Genetic Service of the Garrahan Hospital along 5 year period. We also excluded mycrocephally and MR associated with multiple malformations. We also made neurological and neuropsychological evaluation; and a chromosomical study with FUDR was made to confirm the Fragile X point (X q273) that gives the name to the syndrome. We obtained two groups: A) 42 children (18.7%) from 31 families with positive Fra-X. Range of age 1 to 15 (X 8.31); clinical score 11 to 16(×13.21). MR level in this group was between Moderate (59.3%) and Severe(28.6%). Group B) 183 (181.3%) Fra-X negative patients with a range ae 1 to 18 years(× 7.15) and a clinical score from 1 to 9 (×4.52). MR in this group was between Moderate (44.1%) and Mild (30.4%). According with our results, the most frecuent clinical features (excluding MR and Language disorders) in the Fra-X positive group are: Long face, High palate, Height and cepahlic circumference >p50, Motor stereotypies and Long ears. Macroorchidism, the most consistent clinical feature in the adults, appears in our group only with puberty. We carried out molecular studies evaluating the amplification of the CGG region in the FMR1 gen, with Southern Blot technics in 65 patients (27 Fra-X pos and 38 Fra-X neg>) with high coincidence with the cytogenetic findings. Comparing the clinical, cytogenetical and molecular data, we can stablish a cut off point of 10 in our clinical score, having this score a sensibility of 100% and specifity of 97.4%. The aim of this paper is to introduce a pediatric clinical score to select individuals for cytogenetic and molecular studies for the diagnosis of the Fra-X syndrome, avoiding expensive massive laboratory programs uncompatibles with a rationale resources planning.