A functional ovarian cyst is a fluid-filled sac that forms on the ovary. A sac normally forms during ovulation to hold a maturing egg. Usually, the sac goes away after the egg is released. But if a mature egg is not released or if the sac reseals after releasing the egg, the sac can swell up with fluid. This forms a functional ovarian cyst.

Most of these cysts do not cause symptoms and they go away by themselves. Some cause mild pain. Others can cause severe pain when they rupture or bleed.