The Adverse Event Reporting Ontology (AERO) is designed to facilitate the representation of adverse event reports and their evaluation in relation to clinical guidelines written by various authorities. We take a realist approach in which findings in reports are information artifacts and are related to instances or types that represent patient, symptoms, diseases, etc. We intend that use of this ontology will result in data that interoperates well with OBO Foundry ontologies as we directly use terms from OBO ontologies wherever appropriate.The initial domain for this work are reports of adverse events following immunization (AEFIs). We represent the Brighton case definitions as our guidelines, show how findings can be assessed in terms of Brighton criteria and how diagnoses of adverse events with different levels of certainty can be obtained by use of the OWL reasoner.