With few exceptions, published studies fail to indicate that the DSM-III personality disorders can be distinguished from each other with respect to etiology, prognosis, treatment response, or family history. The Structured Interview for the DSM-III Personality Disorders (SIDP) was developed to improve axis II diagnostic reliability, and hence allow validity testing of axis II. Sixty-three subjects were independently rated by two interviewers using the SIDP. The kappa coefficients for interrater agreement reached .70 or higher for histrionic, borderline, and dependent personalities. While it is impossible to separate the validity testing of the SIDP from validity testing of the DSM-III personality criteria themselves, preliminary results from 102 inpatient SIDP interviews suggest some criterion-based validity with respect to standard personality rating scales and some construct validity with respect to the dexamethasone suppression test.