The sodium (Na) and potassium (K) concentrations were measured in the fluid of 169 breast cysts aspirated from 101 consecutive patients presenting to a single clinic. The Na/K ratio was used to define cysts as either apocrine (Na/K less than 3) or flattened (Na/K less than 3) in type. There was no consistency in the type of cyst for individual patients. Cyst type was not predictive of further cyst development, neither was a particular cyst type associated with the development of, or a history of, breast cancer.