For many people, including health care providers, endometriosis is an unknown disease. It can present in many different ways, making it difficult to diagnose. As a result the diagnosis is often missed and the right treatment cannot be started. This delay can lead to a huge reduction in the quality of life. Based on three cases of endometriosis we show you when to think about this disease. In the cases we describe the most common symptoms of endometriosis are mentioned: pelvic pain, dysmenorrhea and, dyspareunia. It is important to ask a patient with pelvic pain or subfertility about these complaints. If endometriosis is suspected on the basis of these or other complaints, empirical hormonal treatment can be offered to patients who are not trying to conceive at that time.