Dehydroepiandrosterone sulfate (DHAS) has been measured by radioimmunoassay in 100 breast cyst fluids obtained from 82 women. Values ranged from 1.5 to 1,155 microM with a median of 140 microM. These concentrations are in excess of those for plasma but are comparable or less than values for breast secretions obtained by nipple aspiration. Levels of DHAS in cyst fluid were not significantly affected by age, menopausal status, or parity of the subject or by the volume of cyst fluid obtained. In patients with multiple cysts, DHAS values from cysts aspirated from the same breast on the same data were relatively comparable, but wide variations were frequently observed between cysts aspirated on different occasions from the same breast and between cysts from different breasts of the same patient, whether sampled simultaneously or sequentially. Such variability must complicate comparative studies among women.