Ca 125 was assayed in serum and in breast cyst fluid of 78 patients with GCD. Levels of the marker in cyst fluid are generally < 30 U/ml. More significative results were found in relation to cysts relapse. In apocrine cysts relapse levels were higher than 270 U/ml, in serum cysts not relapsed levels were between 31-270 U/ml. Ca 125 could have a significative role in cell differentiation and its control.