BACKGROUND: The relationship between borderline personality disorder (BPD), bipolar disorder (BD), and attention deficit/hyperactivity disorder (ADHD) requires further elucidation.
METHODS: Seventy-four adult psychiatric in- and out-patients, each of them having received one of these diagnoses on clinical assessment, were interviewed and compared in terms of diagnostic overlap, age and sex distribution, comorbid substance, anxiety and eating disorders, and affective temperament.
RESULTS: Diagnostic overlap within the three disorders was 54%. Comorbidity patterns and gender ratio did not differ. The disorders showed very similar levels of cyclothymia.
LIMITATIONS: Sample size was small and only a limited number of validators were tested.
CONCLUSIONS: The similar extent of cyclothymic temperament suggests mood lability as a common denominator of BPD, BD, and ADHD.