This study investigated several aspects of the validity of the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition avoidant personality disorder (APD) construct, with emphasis on the psychometric properties of the diagnostic criteria and the prototype nature of the construct. A sample of 1,058 patients from the Norwegian Network of Psychotherapeutic Day Hospitals was examined by means of exploratory factor analysis, correlation, and diagnostic efficiency statistics, chi(2) analysis, and frequency distribution. The results indicated that APD is a 1-dimensional construct with good internal consistency. The criteria had acceptable diagnostic efficiency; criterion 3 performed poorest. Number of APD criteria showed no distinct threshold between No-APD and patients with APD. Sixty-two different combinations of any 4 APD criteria occurred. It can be concluded that the prototype model fitted the data well and that the APD diagnostic criteria perform well in the current classification system. The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition hierarchy of criteria was not supported.