To define the clinical characteristics of oral ulceration (OU) in Behçet's disease (BD), to allow differentiation from other causes of OU, including aphthous ulcers, by an International Delphi consultation. To develop a clinical guideline on how to recognise BD ulcers.Round 1. 40 clinical images of OU in BD, recurrent aphthous stomatitis (RAS), inflammatory bowel disease (IBD) and mucous membrane pemphigoid (MMP) were shown. Participants answered, independently, which images would be consistent with a BD ulcer. Round 2. The results from marking independently were shown. The panel remarked the questions through iteration process. The images not agreed to be a possible BD ulcer were discarded. Round 3. 10 clinical descriptors that may define BD ulcers were suggested. Participants ranked the level of importance for each descriptor on each image presented. Round 4. Participants re-ranked their level of agreement for each descriptor through iteration process. Whether the clinical pictures would be different from RAS was also explored. A final agreement was reached.This study has shown clear differentiation between BD, IBD and MMP ulcers when defining them by phenotype through clinical images only. On the other hand, no differentiation between RAS and BD ulcers was found. The most important clinical descriptors that define BD ulcers have been agreed.New clinical guidance for Health Care Professionals (HCP) on how to recognise a BD ulcer has been proposed. This should elucidate an earlier diagnosis, quicker access to treatment and control of the disease enhancing patient's quality of life.