PERSONALITY is described in terms of traits shared, to a greater or lesser degree, by everyone. The clinical diagnosis of personality disorder rests on the judgment that several of these traits are present to a quantitatively deviant degree. How reliable and valid is that judgment? The issue of reliability has recently been examined by PRESLEY and WALTON~ in relation to categorical and dimensional classifications of personality disorder. They point to three sources of disagreement among clinicians which lower the reliability of categorical diagnoses: (1) rater bias resulting in varying attention to certain aspects of the patient’s personality; (2) several meanings being attached to the same term; and (3) inadequate definition as to the limits of normal trait variation. Less study has been given the question of validity. SLAVNEY and MCHUGH~ were unable to validate clinical diagnoses of hysterical personality disorder with the Minnesota Multiphasic Personality Inventory (MMPT). They had hoped that such a self-rating instrument might avoid the above-mentioned potential causes of low reliability, and hence low validity, that are inherent in clinical judgments. Most of their patients with diagnoses of hysterical personality disorder were depressed, however, and had an MMPI mean profile which closely resembled that of depressed controls. The authors concluded that the MMPI accurately reflected the similar moods of depressed index patients and controls, but that it failed to assess the traits which physicians apparently recognized when assigning the diagnosis of hysterical personality disorder to one group and not to the other. Emotional lability is one of the traits said to characterize the hysterical personality and it can be directly assessed by serial self-ratings of mood. Such self-ratings have been related to other subjective phenomena,3s5 to endocrine changes-s and to mental illnesses.s-12 We report the use of techniques suggested by some of these studies to examine the hypothesis that self-ratings on variability of mood are positively correlated with self-ratings on hysterical traits. In this way, we hope to demonstrate empirically one aspect of the validity of the concept hysterical personality.