Thirty cases of phyllodes tumour (cystosarcoma phyllodes) of the breast that presented to the Clinical Oncology Unit at Guy's Hospital were reviewed. Tumours were classified as benign, malignant or borderline according to the following histological criteria: mitotic rate, nuclear pleomorphism, stromal overgrowth and tumour margins. In 14 (46.5%) cases the tumours were considered histologically benign, in 11 (36.5%) malignant, and in five (17%) borderline. Recurrence was seen in a similar proportion of patients with tumours classified as benign (21 %) and malignant (18 %) according to histological criteria. Malignant lesions tended to recur earlier. Infiltrating tumour margins were noted in all patients and stromal overgrowth in all hut one in whom recurrence was observed. Risk of recurrence also appeared to he related to tumour size. Only one patient developed distant metastases and died of her disease. Because of treatment variation no conclusion can he made regarding optimal therapy but the importance of adequate clearance, either through wide excision or mastectomy, is emphasized for all phyllodes tumours irrespective of histological features.