The National Szechenyi Library (NSZL) has developed a program that makes it possible to convert UDC numbers of old catalogue records into subject terms. The outcome of the project will be that records of the retrospective catalogue will become searchable both by UDC numbers and with the thesaurus of the National Szechenyi Library. The conversion is based on the data included in the publicly accessible NSZL thesaurus, supported by the RELEX thesaurus management system. This thesaurus already contains a significant quantity of related data (descriptors and UDC numbers as well as UDC indexes and UDC numbers). The conversion is carried out by the JAVA-based program using these relations. This project proves that the UDC numbers of records in retrospective catalogues can be converted into subject terms automatically, without major intellectual investment.