Fragile X syndrome has been recognized as the second most common cause of mental retardation after Down's syndrome, and has been diagnosed by cytogenetic and molecular analyses. Results from cytogenetic and molecular studies by Polymerase Chain Reaction (PCR) techniques in 34 patients suspected of fragile X syndrome (FRAXA) are compared. Results were similar: in five patients with karyotype 46,Yfra(X)(q27.3) the PCR technique also revealed mutation of FMR1 gene. In the other 29 patients who failed to express fragile X chromosome, PCR technique also showed the absence of mutation of FMR1 gene. Although PCR technique guaranteed fast FRAXA search, the cytogenetic study was as efficient as the PCR technique to diagnose fragile chromosome X syndrome in mentally retarded subjects.