Ovarian cyst. Illustration of a human ovary, showing a follicular cyst. At lower centre is the rounded ovary composed of some healthy ovarian tissue (white). The cyst, coloured purple, has produced an enlargement of the ovary, and appears opaque with fluid. At top (red) is part of a fallopian tube through which eggs released by the ovary pass into the uterus (womb). Ovarian cyst is a fluid-filled swelling in the ovary, in this case occurring in an egg-producing follicle. Often there are no symptoms, but a large cyst can cause abdominal discomfort and menstrual irregularities. Most ovarian cysts are non-cancerous. They may go away on their own or may require surgical removal.