Subtelomeric chromosome rearrangements are a significant cause of severe somatic and psychomotor retardation. Their incidence is estimated as 2.1/10000 liveborns and about 30-50% of the reported aberrations were familial. They result in a wide spectrum of clinical signs and most of reported cases are non-repeatable. Clinical verification of mentally retarded patients suspected of subtelomeric rearrangements requires a special molecular-cytogenetic method: subtelomeric fluorescence in situ hybridisation -- FISH. We present a case of submicroscopic subtelomeric aberration of chromosome 13 - der(13)t(X; 13) in a severely mentally retarded child with congenital defects and dysmorphic features. Detailed clinical characteristics of the proband as well as an attempt of a phenotype- genotype correlation of the identified chromosomal aberration were performed.