An ovarian cyst is any collection of fluid, surrounded by a very thin wall, within an ovary. Any ovarian follicle that is larger than about two centimeters is termed an ovarian cyst. Such cysts range in size from as small as a pea to larger than an orange. Most ovarian cysts are functional in nature and harmless (benign). Ovarian cysts affect women of all ages. They occur most often, however, during a woman's childbearing years. Some ovarian cysts cause problems, such as bleeding and pain. Surgery may be required to remove cysts larger than 5 centimeters in diameter.