63 families at-rist of Fragile X-syndrome (FraX) are subjected to Southern blot analysis with the DNA probes Ox1.9 and Ox0.55. Molecular studies have confirmed an initial clinical diagnosis of FraX in 26 families earlier studied cytogenetically and in 11 of 27 families with only some clinical traits of FraX syndrome in proband. Full mutation and premutation condition of FMR-1 gene was ascertained in 34 and rejected in 18 close relatives of probands with the proved FraX syndrome in 37 and families. Four different patterns of pathological alleles are detected at electrophoretograms of DNA samples restricted by endonuclease RcoRI and hybridized to the DNA probe Ox1.9. Prenatal diagnosis of FraX was carried out in two cases at the 1st and 2nd trimester of pregnancy. Perspective of broad application of molecular methods for early diagnostics and prophylactic of FraX syndrome are briefly discussed.