Infants may have breast lumps related to estrogen from the mother. The lump generally goes away on its own as the estrogen clears from the baby's body. It can happen to boys and girls.
Young girls often develop "breast buds" that appear just before the beginning of puberty. These bumps may be tender. They are common around age 9, but may happen as early as age 6.
Teenage boys may develop breast enlargement and lumps because of hormonal changes in mid-puberty. Although this may distress the teen, the lumps or enlargement generally go away on their own over a period of months.
Breast lumps in an adult woman raises concern for breast cancer, even though most lumps turn out to be not cancerous.

Common Causes

Lumps in a woman are often caused by fibrocystic changes, fibroadenomas, and cysts.

Fibrocystic changes can occur in either or both breasts. These changes occur in many women (especially during the reproductive years) and are considered a normal variation of breast tissue. Having fibrocystic breasts does not increase your risk for breast cancer. It does, however, make it more difficult to interpret lumps that you or your doctor find on exam. Many women feel tenderness in addition to the lumps and bumps associated with fibrocystic breasts.

Fibroadenomas are non-cancerous lumps that feel rubbery and are easily moveable within the breast tissue. Like fibrocystic changes, they occur most often during the reproductive years. Usually, they are not tender and, except in rare cases, do not become cancerous later. A doctor may feel fairly certain from an exam that a particular lump is a fibroadenoma. The only way to be sure, however, is to remove or biopsy them.

Cysts are fluid-filled sacs that often feel like soft grapes. These can sometimes be tender, especially just before your menstrual period. Cysts may be drained in the doctor's office. If the fluid removed is clear or greenish, and the lump disappears completely after it is drained, no further treatment is needed. If the fluid is bloody, it is sent to the lab to look for cancer cells. If the lump doesn't disappear, or recurs, it is usually removed surgically.

Other causes of breast lumps include:

Milk cysts (sacs filled with milk) and infections (mastitis), which may turn into an abscess. These typically occur if you are breastfeeding or have recently given birth.
Breast cancer, detectable by mammogram or ultrasound, then a biopsy. Men can get breast cancer.
Injury -- sometimes if your breast is badly bruised, there will be a collection of blood that feels like a lump. These tend to resolve on their own in a matter of days or weeks. If not, the blood may have to be drained by your doctor.
Lipoma -- a collection of fatty tissue.
Intraductal papilloma -- a small growth inside a milk duct of the breast. Often occurs near the areola, the colored part of the breast surrounding the nipple, in women between the ages of 35 and 55. It is harmless and frequently cannot be felt. In some cases the only symptom is a watery, pink discharge from the nipple. Since a watery or bloody discharge can also be a sign of breast cancer, this must be evaluated by your doctor.