Polycystic ovarian syndrome (PCOS), also known as the Stein-Leventhal syndrome, is a condition associated with multiple cysts in the ovaries.

During each menstrual cycle, the ovaries release an egg (ovum) into the uterus. This process is called ovulation and usually occurs once a month. Before the egg is ready to be released, it develops inside a tiny swelling (called a follicle or cyst) on the ovary. Each month, several follicles start to develop, but in most cases only one goes on to mature fully and release an egg.

The ovaries are also responsible for making estrogen, the main female hormone, and small quantities of androgens (male hormones) such as testosterone.

In polycystic ovarian syndrome, many follicles are produced but often none develop enough to release an egg, meaning that ovulation does not take place. The production of hormones is also often unbalanced, particularly with raised levels of testosterone.

Approximately one in ten women have PCOS to some degree, and of those referred for an ultrasound examination, about 25% are found to have polycystic ovaries.

PCOS is now recognized as the most common cause of ovulation failure, leading to infertility, in women who have not yet reached the menopause.