Natural history of cystic disease: the importance of cyst type.

All breast cysts aspirated from a series of 100 patients followed for a minimum period of 2 years were classified on the basis of electrolyte composition as apocrine or flattened, this being the nature of the epithelium lining the two populations of breast cysts. Patients with a single cyst were more than 3 times as likely to have a flattened rather than an apocrine cyst. Multiple cysts, whether simultaneous or sequential in any individual patient, were usually all of the same type, and were more commonly apocrine than flattened. A comparison of the frequency of subsequent cysts in patients whose initial cysts were of either apocrine or flattened type showed further cysts were over 5 times more common in patients who presented with apocrine cysts. These observations suggest that the natural history of cystic disease is closely related to cyst type.