Current systems for classifying personality disorder according to specific categories are unsatisfactory because they do not take account of wide variations in levels of personality disturbance and associated impairment. We review previous attempts to classify personality disorder according to severity and place these findings in the context of work exploring the severity of other mental disorders. On the basis of these findings, we propose a new system for classifying personality-related problems based on severity defined by the extent of personality disturbance, the level of social dysfunctioning, and the impact of the disorder for individuals and society. We recognize that studies using these definitions will need piloting and testing in field trials, but believe that this simplified approach to classifying personality disorder would encourage greater use by clinicians and assist those involved in planning services for people with personality disorder.