Polycystic ovary compared to a normal ovary, illustration. Cysts (grey) are shown in the abnormal ovary (top), with a developing egg (dark pink) in the normal ovary (bottom). Polycystic ovary syndrome (PCOS). PCOS, also known as Stein-Leventhal syndrome, is caused by hormone imbalances and results in multiple cysts forming in the ovaries. A cyst is an abnormal fluid-filled cavity. Ovarian cysts can form when follicles fail to ovulate.