Intellectual disability, defined as an IQ less than 70, occurs in 2-3% of the population and is both a medical and social problem of outstanding importance. Establishing its aetiology is not always easy, but it is a necessary condition in providing genetic counseling for families. Until now, the aetiology of mental retardation remains unknown in about half of the cases. Chromosomal aberrations are found in about 30-40% of moderately and profoundly intellectually impaired. Submicroscopic chromosomal aberrations involving the telomeric regions of chromosomes discovered in the recent years, are responsible for 4-9% cases of idiopathic mental retardation. Identification of very small structural rearrangements in the telomeric regions of the chromosomes requires another strategy than those applied in routine classic methods of chromosomal analysis (fluorescence in situ hybridisation in situ FISH). We present several cases of mental retardation with congenital defects and dysmorphic features which occurred in one family. Submicroscopic aberration in the telomeric regions of chromosomes 7 and 10 was revealed as a cause of mental retardation in this family. That aberration was a result of familial reciprocal translocation which had been not previously identified by routine cytogenetic methods. Detailed retrospective clinical characteristics of the proband and his affected relatives is presented. Variable clinical expression in presented cases in relation to difficulties in diagnosis and genetic counseling is discussed.