Benign phyllodes tumors and fibroadenomas are two types of fibroepithelial tumors of breast that are usually difficult to differentiate. The purpose of this study is to evaluate the proliferative activity of these tumors and to find out if it helps in differential diagnosis. Thirty-one benign phyllodes tumors and twelve cellular fibroadenomas were retrieved from the archives of Pathology Department of Akdeniz University, School of Medicine. Proliferating activity of epithelial and stromal cells were evaluated by using labeling index (LI) of proliferating cell nuclear antigen (PCNA) and Ki-67 antigen by immunohistochemistry. The results were compared with other clinicopathologic findings. There was not any significant difference between the proliferating activity of phyllodes tumor and cellular fibroadenomas. Mean LI of PCNA was 28.01 (+/-22.85) in stromal cells and 56.57 (+/-30.98) in epithelial cells of phyllodes tumor where it was 28.92 (+/-24.02) and 62.53 (+/-32.56) in fibroadenomas. Ki-67 indices were 0.05 (+/-0.19) in stromal cells, 2.65 (+/-12.53) in epithelial cells of phyllodes tumors and 0.0 (+/-0) in stromal cells, 0.43 (+/-0.63) in epithelial cells of fibroadenomas. There was no correlation between the diameter of tumors and proliferating activity in both groups. Proliferating activity, determined by immunohistochemistry with PCNA and Ki-67 antibodies did not reveal significant difference between phyllodes tumor and fibroadenoma.