OBJECTIVE Carcinoembryonic antigen (CEA) is an oncofetal glycoprotein that is widely used as a tumour marker in adenocarcinomas. However, several non-neoplastic conditions, including acute and chronic inflammation and other inflammation-related conditions, are characterised by increased CEA concentrations. Bipolar disorder (BD) ranks seventh among the worldwide burden of non-fatal diseases. Inflammatory biomarkers have been considered as one of the main key pillars of a multifactorial approach for prediction of BD in an at-risk population. BP is accompanied by activation of inflammatory, cell-mediated and negative immunoregulatory cytokines. METHODS We measured the levels of CEA in serum samples from 44 individuals with euthymic BP out-patients and 45 healthy controls. Patients were diagnosed according to the DSM-IV criteria. CEA was measured by an electrochemiluminescence immunoassay. RESULTS The mean serum CEA concentration was 2.36±1.52 and 1.77±0.98 µg/l in patients and controls, respectively. CEA levels were significantly increased in euthymic BP patients when compared with controls (p=0.031). CONCLUSIONS This study suggests that CEA is increased in BD and supports a role for immune activation in the core pathological mechanisms of BP. CEA levels may be a secondary marker for diagnosing BP.