The prototype model, an alternative to the classical view of classification theory, has recently been advocated for the study of the personality disorders. A central assumption of the prototype model is the family resemblance hypothesis. This hypothesis states that the more features of a category that a patient possesses, the more prototypical the patient is of the category. In previous research, results from applying this hypothesis to psychiatric classification have been mixed. The present study investigates the family resemblance hypothesis by examining the relationship between diagnostic agreement and the number of features in personality disorder cases.