Subtelomeric regions of chromosomes are rich in genes; their rearrangements cannot be identified by traditional chromosome analysis. Since these subtelomeric aberrations are responsible for about 7% of cases with mental retardation, their detection is important both from the diagnostic point of view and to prevent recurrence in the family. Subtelomeric chromosomal alterations can be detected by fluorescence in situ hybridization. Based on international criteria, 35 out of 59 patients with mental retardation have been selected. Subtelomeric rearrangements were revealed in 6 patients (5 familial cases, 1 new onset) whereas the subtelomeric FISH result was normal in 29 cases. Deletion of 8pter and duplication of 12pter were detected in 2 patients, while a deletion of 21qter and duplication of the 10pter due to an unbalanced translocation were found in 3 other cases. Finally, a new onset deletion of 3qter was observed in 1 patient. In order to clarify the origin of chromosome aberrations, 12 healthy family members were also examined, 5 of them carried balanced translocations. We concluded that the phenotype is mostly influenced by the size of regions involved in deletion/duplication and - in case of translocations - by the associated chromosomal abnormalities.