BACKGROUND: Using family study data, the following questions regarding the mechanisms of association between personality traits and mood disorders were addressed: 1) Is there an association between unipolar and bipolar mood disorders and personality traits in probands? 2) Are personality traits associated with depression in their 9 to 17 year-old children? 3) Is there an association between parental mood disorders and personality traits in offspring? 4) Are parental personality traits associated with the risk of depression in offspring? METHODS: The study included 50 probands with bipolar and 37 with unipolar mood disorder, 34 healthy controls as well as 178 of their children between 9 and 17 years. Diagnoses were made according to a best-estimate procedure based on a semi-structured interview (DIGS), medical records and family history information. Personality traits were assessed using the Eysenck Personality Questionnaire in adults and the Eysenck Personality Questionnaire Junior in offspring. RESULTS: Personality traits, and in particular Neuroticism, were found to be associated with mood disorders in currently affected as well as remitted probands and offspring. However, there was no association between mood disorders in parents and personality traits in their children, and conversely, parental personality traits were not associated with the risk of depression in offspring. LIMITATIONS: 1) Relatively small proportion of offspring who were still unaffected but likely to subsequently develop mood disorders; 2) cross-sectional design. CONCLUSIONS: The findings were best compatible with the complication or scar hypothesis, which assumes the occurrence of abnormal personality traits as a consequence of previous depressive episodes.