In 614 breasts tumours assumed to be harmless cysts on palpation were aspirated. In 430 cases aspiration was the definitive treatment. In the other 184 cases excision was necessary in order to obtain histology. 11 cysts were radiologically suspect after air filling. In one case excision demonstrated an undifferentiated milk duct carcinoma. 12 cysts had radiologically suspicious areas outside their margins. In 4 cases lobular carcinoma in situ was present. 161 excisions were necessary because no fluid was aspirated at puncture of the tumour. In 10 cases histology showed malignancy: half of these were metastases from a known primary tumour and half were primary breast carcinomas. Out of 17 precancerous states neoplasia could be demonstrated in two cases in addition to papillary and proliferative duct changes. We have classified this neoplasia as lobular carcinoma in situ.