Malignant tumours of the breast occurring as cysts present an incidence from 0.3% to 0.7%. The Authors report their experience during 15 years in 470 patients with carcinoma of the breast. Endocystic carcinoma appeared in 0.6% of patients: the diagnosis is not always possible with cytology, but ultrasound scans make possible to identiphy cysts which have to undergo biopsy because solid areas within the cystic cavity have been detected. In this case surgery is advised even if the cytological examination has been negative.