Endometriosis first presenting in pleural fluid cytology

Most patients with thoracic endometriosis present with catamenial pneumothorax, a rare condition in which recurrent episodes occur within 72 h before or after the start of menstruation. We report a case of thoracic endometriosis presenting with recurrent bloody pleural effusions without pneumothorax diagnosed on pleural fluid cytology. We describe the cytomorphology and immunoprofile of thoracic endometriosis and discuss the differential diagnoses, including neoplastic processes. We also highlight the importance of communication with clinicians for timeliness of diagnosis and treatment, especially when thoracic endometriosis is not suspected.