Fibroepithelial lesions with cellular stroma (FELCS) in breast core needle biopsy (CNB) specimens may result in either fibroadenoma or phyllodes tumor at excision. We evaluated histologic features, proliferation indices (by Ki-67 and topoisomerase II a immunostaining) and p53 expression in 29 cases of FELCS in CNB specimens and correlated these with excision findings in a blinded manner. On excision, 16 patients had fibroadenomas and 12 had phyllodes tumors. All CNB specimens with mildly increased stromal cellularity were fibroadenomas on excision (n=4), and all with markedly cellular stroma were phyllodes tumors (n=4). Among CNB specimens with moderate cellularity (12 fibroadenomas and 8 phyllodes tumors), only stromal mitoses were discriminatory histologically. Stromal proliferation indices were significantly higher in CNB that were phyllodes tumors vs fibroadenomas. Assessment of stromal cellularity, mitoses, and proliferation indices might help determine the probability of phyllodes tumor occurring and guide management of these cases.