HAPI (HL7 application programming interface; pronounced "happy") is an open-source, object-oriented HL7 2.x parser for Java. HL7 (http://hl7org) is a messaging specification for healthcare information systems. This project is not affiliated with the HL7 organization; we are just writing some software that conforms to their specification. The project was initiated by University Health Network (a large multi-site teaching hospital in Toronto, Canada).