Introduction: Phyllodes tumors are fibroepithelial tumors accounting for 1% of primary breast neoplasm with malignant transformations occurring in 10-20% of the tumors. Heterologous osteosarcomatous differentiation is a rare and aggressive variant associated with poor prognosis. Case report: A 38-year-old female presented with a mass that she noticed a month ago with increase in size of mass to 10cm at time of presentation. A biopsy of the mass showed a fibroepithelial lesion with focal areas of stromal overgrowth, accompanied by stromal hypercellularity without evidence of nuclear pleomorphism, mitotic activity, necrosis and heterogenous elements most likely a benign phyllodes tumor. Due to discordance with clinical examination, and large size of tumor she underwent a mastectomy a month after presentation, in the interim period the tumor had grown to 20cm, with the appearance of cystic structures anteriorly. A skin sparing nipple sparing mastectomy was performed, carefully preserving and excising the cystic structure anteriorly. The specimen weighed 950gm.This was followed by an autologous reconstruction with a deep inferior epigastric perforator flap. The following day patient was noted to have venous congestion of the flap and she returned to the operating room for exploration of the flap and an interposition vein graft was performed to revascularize the venous component to a secondary lateral venae comitante of the inferior pedicle. Patient was discharged home the next day. The pathology returned malignant phyllodes tumor with heterologous osteosarcomatous differentiation, pleomorphic stromal cells, increased mitotic activity, all margins free by 0.1cm or less (Fig 1). Patient was taken back to the operating room a month later for reexcision partial mastectomy for margins. This was followed by removal of flap and flat reconstruction. Pathology showed residual malignant phyllodes, nipple and skin negative for involvement and margins negative by 0.2cm or more.Patient did well postoperatively and is currently undergoing radiotherapy Learning point: Phyllodes tumors are fibroepithelial categorized as benign, borderline or malignant based on tumor margins , mitotic activity and stromal cellularity, atypia and overgrowth. A tumor with infiltrative margins, atypia, mitotic activity of >10 per 10hpf, stromal hypercellularity and overgrowth is characterized as a malignant phyllodes tumor. Presence of heterologous sarcomatous differentiation such as liposarcoma, chondrosarcoma or osteosarcoma also indicate malignant transformation. Definitive treatment involves wide surgical excision or mastectomy depending on the size of the tumor. Malignant phyllodes have local recurrence rates ranging from 23 -30%. Studies have shown those with a large tumor and heterologous osteosarcomatous differentiation have greater risk of developing distant metastasis. These tumors spread hematogenously, hence axillary lymph node dissection is not generally indicated. Some studies recommend adjuvant radiation in patient with tumors greater then 5cm and more than 20 mitosis/hpf because of high local recurrence rates.