We present the construction of an expert system (ES) for the diagnosis of Obsessive-Compulsive Disorders (OCD). It concerns an artificial intelligence tool, in Lisp language compatible with any personal computer (PC) with a hard disk. The ES asks the user 50 questions in natural language, on the patient or on a clinical history. It is provided with 115 rules of reasoning. Using single or multivaluate variables, the ES reaches the diagnosis of the Obsessive-Compulsive Disorders or the recommendations of differential diagnosis with related patterns or involucred with obsessive pathology: phobic, affective, schizophrenic and Gilles de la Tourette disorders. Finally, the perspectives for the utilisation of the ES in psychopathology are disscussed, in conjunction with the 2 serious problems created, design difficulty and user acceptance.