In view of a psychiatric epidemiological project, a thought is given to psychiatrie case definition. After a review of basic epidemiological concepts, stressing the need of a definition for caseness, three opinions are discussed. The uselessness of a general idea of mental disease and the desirability of specific diagnosis with predictive capacity, are explained, with respect to professional case definition. The PSE is presented as a case-finding instrument, capable to provide for an operational definition of mental desease. The second definition, called neediness, refers both to a professional inclination to help, starting from an expert view on the problem, and the (lay-)want to be helped. When a case is registrated (the third definition) lay and professional mostly agree about the necessity of help. Several factors play a part in this decision, that should be a condition for treatment. Between these three concepts a comparison is made, from which fourteen possible case definitions result. Clarification of this subject should come from research using a multi-axial concept of caseness.