Community-based epidemiological studies of psychiatric disorders provide important information about the public health burden of these problems; however, because seeking treatment is related to a number of clinical and demographic factors, studies of the frequency and correlates of psychiatric disorders in the general population should be replicated in clinical populations to provide the practicing clinician with information that might have more direct clinical utility. Diagnosing co-occuring personality disorders in psychiatric patients with an Axis I disorder is clinically important because of their association with the duration, recurrence, and outcome of Axis I disorders. This article reviews clinical epidemiological studies of personality disorders and finds that in studies using semi-structured diagnostic interviews, approximately half of the patients interviewed have a personality disorder. Thus, as a group, personality disorders are among the most frequent disorders treated by psychiatrists.