Research on phenotypic markers of vulnerability to bipolar disorder has focused on the identification of personality traits uniquely associated with the illness. To expand knowledge in this area, we compared Cloninger's seven temperament and character dimensions in 85 euthymic/subsyndromal bipolar I inpatients and outpatients and 85 age and sex matched community controls. We also examined associations between Cloninger's personality traits and mood state in the patient group. Bipolar subjects were administered the Structured Clinical Interview for DSM-IV (SCID), Hamilton Rating Scale for Depression, and Clinician-Administered Rating Scale for Mania. Controls received the SCID, a family psychiatric history questionnaire, and urine toxicology screen to confirm healthy status. Both groups competed the 240-item Temperament and Character Inventory (TCI). A multivariate analysis of covariance, accounting for demographic factors, was conducted to compare the groups on the TCI. Bipolar I patients scored higher on harm avoidance, lower on self-directedness, and higher on self-transcendence compared to controls. Harm avoidance and self-directedness were correlated with residual depressive symptoms positively and negatively, respectively; persistence was correlated with residual manic symptoms; and selftranscendence was correlated with residual psychotic symptoms in patients. The results indicate that bipolar I subjects do possess personality traits that are significantly different from non-ill individuals. However, only a prospective, longitudinal study may determine whether these traits mark a vulnerability to the disorder, or represent the scarring effect of affective episodes and chronic subsyndromal symptoms.