OBJECTIVES: Bipolar disorder (BD) is a chronic psychiatric disorder with a high prevalence of obesity. There are a number of hypotheses regarding the association between obesity and BD. One involves common neurobiological abnormalities, such as dysfunction in the hypothalamic pituitary adrenal axis and changes in secretions of orexigenic and anorectic peptides. The purpose of this study was to evaluate the blood levels of agouti-related peptide (AgRP), obestatin cortisol, and corticosteroid-binding globulin (CBG) and metabolic parameters in patients with euthymic BD, and to compare these to those of healthy controls. METHODS: Twenty-nine outpatients with BD type I admitted to the psychiatric clinic were consecutively enrolled and compared with 25 sex- and body mass index (BMI)-matched controls. RESULTS: There was a significant difference in AgRP, cortisol, and CBG levels between patients and the controls (p = .005, .021, and .034, respectively). AgRP and CBG did not correlate with any parameter in BD patients, but cortisol correlated with BMI. CONCLUSIONS: We conclude that BD patients have higher levels of AgRP, cortisol, and CBG than healthy controls with similar BMIs. This may represent a new insight into the neurobiology of BD.