What is the likely cause of a young woman to have a breast lump?

Fibroadenoma: The most common breast tumor is a fibroadenoma. This is a benign tumor which may be present as early as the teen years. As women age, breast cysts may develop, which are benign fluid-filled lumps (peak incidence ages 35-50). It is often very difficult to distinguish a lump from normal "lumpy-bumpy" tissue. When in doubt, it is best to see your primary care physician or gynecologist.