The case of a 1.5 year old girl with clinical traits of craniofacial dysmorphy, hypotonia, polydactyly and moderate mental retardation is presented. Routine cytogenetic study revealed the presence of a large additional chromosomal fragment associated with the nucleolus organizing region on one of chromosomes 13. The banding pattern suggested the additional fragment was a part of the long arm of this chromosome. The set of clinical symptoms was only partly consistent with those characteristic for trisomy 13q2 and 3. Application of the FISH technique with a chromosome 13 specific library enabled final confirmation of the origin of the extra chromosome fragment from the long arm of chromosome 13. The presented case proves the usefulness of the FISH technique for the diagnosis of chromosomal aberrations and for adequate clinical interpretation of cytogenetic results.