Although DSM-III personality disorder criteria have demonstrated acceptable reliability, the question of validity has not been adequately addressed. A first step in establishing the validity of diagnoses is to establish the validity of the criteria used to assess each diagnosis. The content validity of diagnostic criteria was investigated in relation to the larger set of potential criteria culled from the psychiatric literature. For each DSM-III axis II diagnosis, a panel of clinicians rated how prototypical each potential criterion was of the diagnosis in question. The results reveal problems with the organization and content of the criteria for most diagnoses. Many DSM-III criteria are composed of several statements linked by conjunctions or disjunctions. These component statements often received markedly different ratings, suggesting that criteria should be single statements. For most diagnoses, traits not included in DSM-III received higher ratings than did some DSM-III criteria. Suggestions are made to improve the distinctiveness and content validity of paranoid, schizoid, antisocial, borderline, avoidant, dependent, and compulsive personality disorders. The results for schizotypal personality disorder suggest that many clinicians are uncertain about this diagnosis. These findings provide a systematic way to modify definitions that contrasts with the more arbitrary ways in which diagnoses have previously been defined and redefined.