Background: The etiology in bipolar disorder has not been fully understanding. There are limited data regarding the relationship between the permeability of intestinal and blood-brain barrier (BBB), and bipolar disorder etiology. Zonulin is regarded as a non-invasive biomarker for intestinal permeability. Claudin-5 is an important part of BBB permeability. In this study, we assumed that there may be a deterioration in serum zonulin and claudin-5 levels in patients with bipolar disorder and this may affect the severity of the disease.