This Australian Handbook comprises sufficient detail and discussion for the implementation of an HL7 based system for pathology messaging. It is intended that it be used as an implementation guide in conjunction with HL7 V2.3.1 and AS 4700.1.Pathology messaging implementation comprises both orders and results. The guidelines are presented in three stages:(a) Overview of HL7 structure and operation.(b) Discussions on issues relating to pathology messaging in Australia.(c) Detailed definition of HL7 message segments.The guidelines clarify the way a particular situation should be interpreted or handled. Implementers must adhere to the field definitions of AS 4700. However, this guide describes methodologies and field structures and ways in which the content can be developed.With regard to the actual content of some fields it is not mandatory to use the methods shown herein. They are suggestions and are used in the examples. Individuals or businesses wishing to use or develop their own proprietary methods should ensure that they meet the reasoning outlined in the relevant section. That is, they are not at cross-purposes with methods implemented by others with whom they are communicating. The information included in this guide includes recommended actions, structures, content and values that are the preferred options. Using them increases the likelihood that conflicts can be avoided.One aspect included in this guide and not covered by the HL7 protocol relates to the transfer of data between the supplier and consumer. In many cases this will occur between private surgeries and private pathology laboratories. The most likely scenario for such communication is by using data file transmissions.Relevant issues relating to file processing, and data encryption are discussed in some sections. Where this is not applicable to a real-time installation these sections can be ignored.The processes of encryption and transmission are not defined nor discussed in this guide.