Phyllodes tumours are rare accounting for 0.3-1% of all breast tumours. Malignant phyllodes tumours make up 20% of all phyllodes tumours. A 64 year old woman who had never had a mammogram was referred by her GP for a large mass of her breast. This had been present as a small golf ball sized mass for 5 years but had grown rapidly over the last year. Imaging showed a 20cm tumour invading into the pectoralis major muscle, and biopsies revealed a spindle cell lesion. After multidisciplinary discussion, the patient proceeded for a radical mastectomy including excision of pectoralis major muscle, and application of a vacuum dressing. Post-operative histology identified the tumour as being a malignant phyllodes tumour with clear margins. The patient then underwent reconstructive surgery to cover the defect of the chest wall. The mainstay of management for malignant phyllodes tumours is complete surgical resection aiming for 1cm margins. Radiotherapy can be considered to reduce local recurrence. Ten year survival rates are good in the absence of metastatic disease.