Aim: The aim of this report was to describe the steps of a diagnostic protocol used to recognise the presence of a third root canal in first and second maxillary premolars at an early stage. Materials and methods: A total of 43 maxillary first premolars and 51 maxillary second premolars were endodontically treated in a private practice between 1999 and 2004 after having been examined clinically and radiographically. Results: A third root canal was present in 4 out of the 43 first maxillary premolars (9.3%) and in 1 out of the 51 (1.9%) second maxillary premolars that were endodontically treated. All of the five cases were diagnosed preoperatively. The early interception of an anatomical variant allows the clinician to choose the best operative strategy for a specific tooth in terms of access cavity design, canal shaping and cleaning, filling and post-endodontic restoration. Conclusions: Radiographic and clinical examinations of maxillary premolars are essential for early diagnosis of the presence of a third root canal. A careful examination of the pulp chamber floor can help to find the orifice of a third root canal intra-operatively. Schlagwörter: anatomical variants, maxillary premolars, radicular anatomy, three-canal maxillary premolars