The construction and maintenance of a medical thesaurus is a non-trivial task, due to the inherent complexity of a proper medical terminology. We present a methodology for transaction-based anomaly detection in the process of thesaurus maintenance. Our experiences are based on lexicographic work with the MorphoSaurus lexicons, which are the basis for a mono- and cross-lingual biomedical information retrieval system. Any “edit”or “delete” actions within these lexicons that undo an action defined earlier were defined as anomalous. We identify four types of such anomalies. We also analyzed to which extent the anomalous lexicon entries had been detected by an alternative, corpus-based approach.