There was a need for a definition of mental disorder in the preparation of the Third Edition of the American Psychiatric Association's Diagnostic and Statistical Manual (DSM-III). Decisions had to be made on a variety of issues that seemed to us to relate to the fundamental question of the boundaries of the concept of mental disorder. Without some definition of mental disorder, there would be no explicit guiding principles that would help to determine which conditions should be included in the nomenclature, which excluded, and how conditions included should be defined. This article describes a definition of and criteria for medical disorders. The definition of mental disorder proposed here is merely a subset of the definition of medical disorder, it contributes to the continuing debate concerning the appropriateness of the medical model as applied to psychiatric disturbance. The proposed definition is: “A medical disorder is a relatively distinct condition resulting from an organismic dysfunction which in its fully developed or extreme form is directly and intrinsically associated with distress, disability, or certain other types of disadvantage. The disadvantage may be of a physical, perceptual, sexual, or interpersonal nature. Implicitly there is a call for action on the part of the person who has the condition, the medical or its allied professions, and society. A mental disorder is a medical disorder whose manifestations are primarily signs or symptoms of a psychological (behavioral) nature, or if physical, can be understood only using psychological concepts”. Operational criteria are proposed to provide sufficient evidence for both an organismic dysfunction and justification for societal recognition of the appropriateness of the sick role. A condition must meet all criteria to be considered a medical disorder. Each of the criteria follows with explication of key concepts.