The International Society for the Study of Vascular Anomalies (ISSVA) is a multidisciplinary international society of physicians, scientists, and health care providers united by an interest in vascular anomalies. The Society aims to promote the highest standards of care for patients with vascular anomalies by advancing clinical and scientific knowledge concerning causes, diagnosis and treatment, and by education of physicians, health care providers, patients and the community. The Society encourages the free flow of information between its members and interested groups, through workshop meetings and teaching programs, and by the dissemination of a classification scheme and pertinent scientific data.The updated classification was approved at the May, 2018 General Assembly in Amsterdam, the Netherlands and can be found at https://www.issva.org/classification.What is the ISSVA Ontology?The ISSVA ontology has been created to represent a sharable semantic data model of the ISSVA classification. This ontological representation of the ISSVA classification is machine-readable and facilitates interoperability between systems that capture data about vascular anomalies. It is structured hierarchically and contains all concepts of the ISSVA classification and the relationships between them. The ontology is available in the Web Ontology Language (OWL) format, and can be browsed and downloaded online: https://bioportal.bioontology.org/ontologies/ISSVAHow to Use the ISSVA Ontology?The ISSVA ontology does not substitute the ISSVA classification, instead, it should be used as an additional component to facilitate interoperability. Every concept in the ISSVA ontology has a unique code that should be attached to the corresponding data element. In the context of databases, the ISSVA ontology codes could be added as an extra column to the existing data. Mappings to Other Ontologies?The ISSVA ontology contains machine-readable mappings to equivalent concepts in other ontologies, namely: the Orphanet Rare Disease Ontology (ORDO), SNOMED Clinical Terms (SNOMED CT), National Cancer Institute thesaurus (NCIT), and the Human Phenotype Ontology (HPO). This means that data annotated with codes from the ISSVA ontology are interoperable with data annotated with the aforementioned ontologies, without any extra effort, as long as there exists a mapping between the concepts.Issue ReportingPlease contact ISSVA at info@issva.org in case you have any issues and/or feature requests for the ISSVA ontology. The ontology can then be updated accordingly.