Cholethorax is a bilious pleural effusion caused by a pleurobiliary fistula or leakage of bile into the pleural space. Most cases of cholethorax arise from a complication of abdominal trauma, hepatobiliary infection, or invasive procedures or surgery of hepatobiliary system. However, we experienced a case of a patient with cholethorax of unknown origin. There was no evidence of pleurobiliary fistula or leakage of bile from the hepatobiliary system although we examined the patient with various diagnostic tools including chest and abdominal computed tomography, endoscopic retrograde cholangiopancreatography, tubography, bronchofiberscopy, hepatobiliary scintigraphy and video-assisted thoracoscopic surgery. Herein we report a case of cholethorax for which the specific cause was not identified. The patient was improved by percutaneous drainage of pleural bile.