The validity and reliability of the diagnosis of bipolar II disorder has been questioned by means of comorbidity with nonaffective disorders, including substance abuse, personality disorders, and anxiety disorders. This study examined the comorbid diagnosis of a sample of bipolar II patients, comparing patients with comorbidity and those with "pure" bipolar II disorder. Forty Research Diagnostic Criteria (RDC) bipolar II patients were assessed by means of the Schedule for Affective Disorders and Schizophrenia, Lifetime Version (SADS-L) and Structured Clinical Interview for DSM-III-R axis I (SCID-II) for personality disorders. Patients fulfilling RDC criteria for any psychiatric disorder (except personality disorders) or DSM-IV criteria for any personality disorder were compared with patients without comorbidity. For practical reasons, cyclothymia was not considered as a comorbid diagnosis. Half of the sample had lifetime comorbidity with other psychiatric disorders, mainly personality disorders (33%), substance abuse or dependence (21%), and anxiety disorders (8%). However, only the rates of suicidal ideation (74% v 24%, chi square [chi2] = 9.03, P = .003) and suicide attempts (45% v 5%, chi2 = 8.53, P = .003) were significantly different between patients with and without comorbidity. In summary, although the rates of comorbidity are relatively high in bipolar II disorder, most clinical and course variables are strikingly similar in patients with and without comorbidity except for suicidal behavior, suggesting that comorbidity does not reduce the validity of the diagnosis of bipolar II disorder.