Fragile X syndrome is one of the most common reasons for human hereditary mental retardation. It is associated with the expansion of CGG repeats in the 5'-untranslated region of the FMR1 gene, which results in the suppression of its expression and the development of the disease. At present, methods based on PCR and Southern blot analysis are used for diagnostics of the fragile X syndrome. The presence of a fragile site FRAXA on the X chromosome is typical for patients with this pathology. We developed a method of visualizing this site in cell cultures obtained from patients using the fluorescent in situ hybridization (FISH) and the combination of two probes. The method allows one to detect five types of signals on the X chromosome, three of which are normal, while two are associated with the emergence of fragile site FRAXA. An analysis of the distribution of all signal types in cell lines from healthy individuals and patients with fragile X syndrome demonstrated that the method allows one to determine differences between lines with a high statistical significance and that it is applicable to detecting cells that are carriers of the syndrome.