A chocolate cyst is a type of growth that occurs on the ovaries. It is caused by endometrial cells, which usually form the lining of the uterus, detaching and traveling to the outside of the ovaries, where they attach, grow, and sometimes replace some of the ovarian tissue. Also called endometriomas, they are filled with old blood that forms a thick, brown substance that resembles chocolate.

Women with endometriosis are at risk of developing a chocolate cyst. This condition causes endometrial cells, normally only found inside the uterus, to grow on the outside of it as well. Once these groups of external endometrial cells have formed, there is the potential they can come loose and migrate to the ovaries. Chocolate cysts are sometimes referred to as endometriosis of the ovaries.

The main symptom of a chocolate cyst is pelvic pain, though smaller cysts may be asymptomatic. Hormone fluctuations during the menstrual cycle affect their size, so pain may come and go over the course of a month. They typically do not cause infertility directly, but may interfere with ovulation. Sometimes these types of cysts will rupture, which causes the chocolate-like material to leak into the pelvic cavity, where it will get into the spaces between organs there. A rupture can be very painful and the spill of material may cause affected organs to bind together.

Diagnosis of a chocolate cyst may include several steps. Typically, the patient's history will be reviewed to determine if she has had cysts previously, or if she has endometriosis. A doctor may perform a physical examination to try an locate masses on the ovaries. A blood test called a CA 125 will then typically be used to look for evidence of a cyst; the same test is also used in the diagnosis of ovarian cancer, however, so a biopsy may also be necessary for true confirmation. An ultrasound, typically performed transvaginally, can also help confirm the presence of cysts.

Unless a chocolate cyst becomes painful or threatens to rupture, treatment may not be necessary. In cases where the condition becomes problematic, surgery may be required. The preferable option is to remove only the cyst using a procedure called a cystectomy. In extreme cases where the cyst is too large or there are too many of them, removal of the ovary may be required. This procedure is known as an oopherectomy.