Fra (X) or Martin-Bell syndrome is the most common X-linked mental retardation with an incidence of 1/1000-2000 newborns. Chromatid break, double chromatid break or total loss of distal part of X chromosome (which occurs most often inside the C positive band q 27.3) is demonstrated in most male hemizygotes as mental retardation and specific phenotypic features. Fra (X) syndrome is proved in the cultured lymphocytes or fibroblasts with special cytogenetic methods. The prenatal diagnosis is possible by examining of amniotic fluid or the lymphocytes from the umbilical cord. We report two families with fra (X) syndrome. In the first one, 6 year- and 9-month-old boy with mental retardation and characteristic phenotypic features has been recognized as the carrier of fra (X) syndrome and after that his 4-year-old brother with similar symptoms. In the second family, there is a severe mentally retarded 3-year-old boy with fra (X) syndrome who besides typical phenotipic changes also exhibits symptoms of autism. The percentage of the cells with fra (X) chromosome in our patients (30%, 28%, 18%) is not correlated with the degree of their mental retardation. The mothers of our patients are the heterozygous carriers of the syndrome (3% and 1.5% fra (X) chromosome).