A 43-year-old woman felt chest discomfort and was diagnosed with right pleural effusion by performing chest X-ray by a nearby clinic. The pleural effusion cytodiagnosis was positive. She was admitted to our hospital for intensive examination and treatment. A chest pelvis computed tomography (CT) scan revealed no signs of primary lesions. However, the patient reported past history of a myoma of the uterus. Furthermore, ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography was performed for an intensive examination. Because FDG was accumulated in the previously affected part of the myoma of the uterus, the possibility of uterine cancer was also considered. Pathological investigations revealed adenocarcinoma on the pleural effusion cell block. Moreover, immunostaining of this block showed estrogen receptor (ER) (+), progesterone receptor (PgR) (+) staining. Because a hormone receptor-positive adenocarcinoma was diagnosed, uterine, ovarian, and breast cancers were considered as distinct possibilities. Although cytology of the endometrium was performed several times, a malignant tumor was not identified. This patient exhibited malignant pleural effusion, which did not result in metastasis-to-axillary lymph nodes and was shown to be hormone receptor ER (+) and PgR (+). It was regarded as cancer of an unknown primary origin, with a possibility of occult breast cancer.