This study investigates the reliability, validity, and diagnostic efficiency of the Rorschach Perceptual-Thinking Index (PTI) in relation to the accurate identification of psychotic disorder (PTD) patients. The PTI is a revision of the Rorschach Schizophrenia Index (SCZI), designed to achieve several criteria, including an increase in the diagnostic utility of the Rorschach for assessing thought disorder and to increase clinical focus on the dimensional aspects of impaired perceptions and thoughts (Exner, 2000a; 2000b). Seventy-eight patients who met DSM-IV criteria for a PTD or Axis II disorder [PTD = 33; borderline personality disorder (BPD) = 23; cluster A personality disorders (CA) = 9; cluster C personality disorders (CC) = 13] and 40 non-patients were compared on the PTI. The results of this study indicate that the PTI can be reliably scored and is internally consistent. In addition, the PTI can be used to effectively differentiate PTD patients from a non-patient sample as well as from personality disorder patients characterized by moderate to, at times, severe perceptual thought disorder. Finally, the PTI can also be employed for classification purposes in ways that are clinically meaningful in the diagnosis of a psychotic disorder. Conceptual and methodological issues are discussed in relation to the assessment of psychosis.