The fragile X syndrome, the most common form of inherited mental retardation, is characterized by unique genetic mechanisms, which include amplification of a CGG repeat and abnormal DNA methylation. Direct DNA analysis of fragile X mutations has already shown its clear superiority for postnatal and prenatal diagnosis of the disorder and for carrier detection. In this, paper the authors report on the results of DNA analysis in families with familial mental retardation. They present the various alternatives (probe/enzymes combinations) for Southern blot based diagnosis and protocols which gave optimal results for detection of patients segregating for fragile X syndrome. Totally, 36 members from 10 families were analyzed by Southern blotting, including 18 mentally affected patients. No CGG expansion was detected in 9 clinically affected patients of 5 families. Expansion of the CGG repeats was found in 9 clinically and cytogenetically affected males, in 5 unaffected carriers of premutation, and in 1 carrier of full mutation in the remaining 5 families. Carriers represented mothers of the patients. These results correlated with cytogenetic and clinical expression of fragile X syndrome. The application of the method for diagnosis of the disease is discussed. (Tab. 2, Fig. 3, Ref. 21.)