Ovarian follicular cysts are the most common type of neoplastic lesion of the ovary. Ovarian follicular cysts are caused by atresia of the ovarian follicles, which occurs every month in adults when some follicles die with the egg. Cystic atresia is often small and multiple, and some cysts can increase in size significantly, resulting in pelvic discomfort and torsion in a few cases. When the cyst is large, the granulosa cells may disappear due to compression by the cyst fluid, and the cyst wall is then composed only of inner vesicular cells or glassy connective tissue. In the latter case it cannot be distinguished from non-follicular cysts and is therefore called a simple cyst. In rare cases, the cyst wall may be covered with functionally active granulosa and/or vesicular cells and estrogen-rich cyst fluid, which can lead to excessive endometrial hyperplasia and irregular uterine bleeding. In the elderly, especially in postmenopausal women, there is a risk of malignant transformation if such cysts persist for a long time. In addition, follicular cysts are most common in girls with precocious puberty, and in many cases this cyst is treated surgically as a granulosa cell tumor. However, most of these cases regress spontaneously. Ovarian follicular cysts often resolve on their own, but sometimes they can persist.