Ca 15-3 was assayed in serum and in breast cysts fluid of 78 non neoplastic patients presenting Gross Cysts (GCD). In apocrine cysts and in the mixed type (serous and apocrine, class III) relapse was indicated by high serum and intracystic levels of the marker. Ca 15-3, therefore, may discriminate in a group of cysts a higher cellular resistance as well as an increased cell proliferation. Results suggest an important role of the marker in the follow up of patients with GCD and for the early detection of cyst relapse.