Despite their widespread use, the terms "syndrome", "disease" and "diagnosis" are sometimes utilized improperly and ambiguously, compounding the complexities of medical knowledge representation. The definitions and illustrative examples provided here will be useful for developers of diagnostic expert systems. Description of the Problem Representing medical knowledge is a highly complex endeavor. The improper use of the terms "syndrome", "disease" and their relations to "diagnosis" is one of the difficulties with which medical informaticians must deal, especially when developing expert systems to support diagnoses. Although ubiquitous in medical and lay discourse, the term "disease" has no unambiguous, generally accepted definition. How-ever, most of those using this term allow themselves the comfortable delusion that everyone knows what it means. Only sparse and fragmented literature could be found regarding this issue.