Breast cysts, tiny accumulations of fluid, are the most common cause of benign (non-cancerous) breast lumps in women between age 30 and 50. Simple cysts are typically round or oval and have smooth edges.  Complex cysts can be filled with debris and may sometimes require aspiration to confirm that they are indeed benign cysts.  Both single and multiple cysts are very common.  The exact causes of cysts are not known, but they do tend to change with hormonal variations, either during normal menstrual cycles or from post-menopausal hormone replacement therapy.

Cysts often fluctuate with the menstrual cycle and are particularly common during periods of hormonal irregularity. Clinically, cysts are usually well demarcated from the surrounding breast tissue. They are characteristically firm and mobile. Cysts that have filled rapidly may be tender.

On physical examination, it is often difficult to distinguish a cyst from a solid mass. Ultrasonography or aspiration must be used to establish a definitive diagnosis. Cysts require surgical biopsy only if the aspirated fluid is bloody, the palpable abnormality does not resolve completely after the aspiration of fluid or the same cyst recurs multiple times in a short period of time.

Patients with a solitary breast cyst should be reexamined four to six weeks after cyst aspiration to determine if the cyst has recurred.