Thoracic endometriosis, rarely encountered, is characterized with the localization of functional endometrium tissue in pleura, lung parenchyma or tracheobronchial system. A 28 years old female patient visited our clinic with complaints of cough and shortness of breath for the last two months. Right-sided massive pleural effusion was detected in the chest radiography and thorax computed tomography. Exudative fluid was aspirated with a haemorrhagic appearance on thoracentesis. Cytology was evaluated as suspicious. "Signet ring cells" were reported in pleural biopsy. Diagnostic biopsy was performed by video-assisted thoracic surgery (VATS) on the patient whose fiber-optic bronchoscopy was normal. The histopathological diagnosis was reported as "pleural endometriosis". Chemical pleurodesis was applied with asbestos-free chalk. The thorax HRCT (high resolution computed tomography) performed during menstruation was normal. Thoracentesis was needed 3 times for recurrent pleural effusion in the follow-ups and then parietal pleurectomy was performed. The patient is in postoperative 10th month follow-up and evaluated as normal clinically and as radiologically.