What is gynecomastia? Gynecomastia refers to a condition in which males develop swelling of breast tissue. It should not be confused with pseudogynecomastia, in which adipose (fat) tissue takes on the appearance of breasts. Gynecomastia is not uncommon, especially in pubescent males and male infants. Gynecomastia arises from changes in androgen and estrogen levels, which control the development of male and female characteristics, respectively. Gynecomastia often starts as a tender lump below the nipple. Lumps are generally small in size. The development of breast tissue can occur on one or both sides, and growth can occur unevenly. In certain cases, such as in infants, galactorrhea (milk production) can occur along with gynecomastia. Gynecomastia can arise from heredity, lifestyle, and developmental factors. Gynecomastia may occur in up to half of males undergoing puberty. In addition, it is common in bodybuilders, those who use steroids, and marijuana users. It may also be related to conditions such as thyroid imbalance. In most cases, gynecomastia will resolve spontaneously. In situations in which bodybuilding or recreational drug use leads to breast tissue development, addressing the underlying cause of gynecomastia will often resolve the condition. In rare cases, however, gynecomastia may require hormone therapy or breast reduction surgery. While gynecomastia is common and is generally not serious, seek prompt medical care for symptoms that may indicate breast cancer, such as breast development on only one side, firm lumps within the breast, ulcers of the breast, or bloody nipple discharge. Seek prompt medical care for prepubescent males with symptoms of gynecomastia, as these may indicate a serious underlying condition.