It is currently known that Fragil X syndrome represents the main cause of mental retardation of chromosomal origen after Down syndrome. Its incidence is 1:2000 male newborns and 0,4:2000 female newborns. Phenotypic abnormalities in males include: mental retardation, facial dysmorphies, connective tissue anomalies and macroorchidism. A variable number of affected patients showed fragility in Xq27.3 when cytogenetics studies are performed under folate deficient conditions. At molecular level it has been demonstrate that there is an amplification of the repeat CGG triplet. The number of mutated cells and its distribution in different tissues could be related with the diversity of phenotype severity seen in this syndrome. We present then years of clinical and cytogenetic experience in the fragil X syndrome in patients controlled in a Neurogenetical Diseases outpatient clinic. This syndrome was suspected in 289 patients, 31 of them had a cytogenetic abnormality: Xq27.3 fragil site(19); sexual chromosomeaberration (47,XXY(4), mos45,X/48XYYY(1) and autosomal abnormalities in seven of them (46,XY,t(12p;18p)-46,XX, inv del(3)-46,Xy,fra(17)-46,XY,der(12)- 46,XY,mar(9) and 2 46, XX,t(7;20)). We analize thephenotype in relation with the genotype and emphasize the absolute necessity of stablishing molecular biology techniques in order to make diagnosis, and also for genetic characterization of chilean population in relation whith this syndrome.