Thoracoscopy is a useful method for diagnosing plural lesions. We diagnosed 3 cases of ovarian adenocarcinoma by thoracoscopy. All patients were middle-aged and elderly women. Initial examination revealed intrathoracic lesions including pleural effusion, but intraperitoneal lesions were unclear. In all cases, adequate samples of tumor cells obtained using thoracoscopy were used in the identification of tumor origin. Elevated serum levels of cancer antigen 125 (CA-125) and Krebs von den lungen-6 (KL-6) were observed in 2 of the 3 patients. Diagnoses of ovarian cancer were based on immunohistochemical examinations and clinical course. This report describes the diagnostic usefulness of thoracoscopy and serum KL-6 in cases of ovarian cancer with predominantly intrathoracic lesions.