A 31-year-old woman was admitted to our hospital with complaints of 4 months of breathlessness. She has noticed the onset of breathlessness 4 months ago and was admitted to the local hospital. Routine blood examination revealed a moderate degree of anemia (Hemoglobin 75g/L). A massive right-sided PE was identified on chest computerized tomography (CT) scans. She underwent thoracentesis. 3,000 ml hemorrhagic fluids were drained and contained no malignant cells. Breathlessness was relieved. She was discharged with a diagnosis of hemorrhagic pleural effusion of unknown origin. The patient was transferred to our hospital because of recurrence of dyspnea 2 months ago. She was a non-smoker and had a 4-year history of infertility and bilateral ovarian chocolate cysts. Two times of laparoscopic ovarian cystectomy have been done in the past 4 years. She had regular periods every 28 days, lasting 7 days with severe dysmenorrhoea. The physical examination was consistent with massive right-sided PE. A repeated Chest CT scan revealed massive right PE (Figures 2A,B). Thoracentesis and chest tube drainage were performed. Pleural aspirates revealed a deep red-colored hemorrhagic appearance (Figure 2C). Blood laboratory findings revealed anemia: hemoglobin 75g/l, red blood cells 3.9 x 1012/L, hematocrit 31%. Analysis of the pleural fluids showed the following values: red blood cell 260,000/mm3, hematocrit 2%, white blood cell 190/mm3, mononuclear cells 84%, total protein 50 g/L, albumin 33g/l, lactate dehydrogenase 260 IU/l, adenosine deaminase 15.5 IU/l, glucose 5.2 mmol/L, T-SPOT.TB 0 spot forming cells/106 PBMCs, normal carcinoembryonic antigen, negative tuberculosis/non-tuberculous mycobacterium DNA amplification, negative acid-fast bacilli staining, and negative culture for bacteria, fungi, and mycobacteria. Cytological examination of liquid based cytology (ThinPrepTM) smears showed scattered clusters of endometrial glandular cells (Figure 3A). Immunohistochemical staining of sediment cell blocks for estrogen receptor and progesterone receptor showed nuclear positivity (Figures 3B,C). A diagnosis of endometriosis-related PE was made. The patient was prescribed leuprolide (3.6 mg intramuscularly every month). At 14-month follow-up, there was no recurrence of breathlessness. Chest radiography showed a little right-sided pleural effusion. The patient underwent HBSO because of severe dysmenorrhoea, menorrhagia, and infertility. Oral Tibolone Tablets were prescribed (1.25 mg every day). Spontaneuos right-sided hydropneumothorax occurred 3 months later. The patient discontinued Tibolone tablets and the symptoms of cough and dyspnea improved. She declined repeated chest CT, thoracentesis, and thoracic surgery.