The present study was undertaken in order to elucidate the question of whether the distribution of stromal CD34+ fibrocytes and smooth muscle actin (SMA)-reactive myofibroblasts differs between benign and malignant lesions of the breast. We investigated a total of 31 ductal carcinomas and 27 specimens with benign lesions of the breast (ductal hyperplasia, sclerosing adenosis, fibroadenoma, phyllodes tumor) and compared the distribution of CD34+ fibrocytes and SMA-reactive myofibroblasts. The stroma of normal breast tissue contained CD34+ fibrocytes, whereas SMA-reactive myofibroblasts were absent. All benign breast lesions exhibited stromal CD34+ fibrocytes and few lesions (fibroadenomas and phyllodes tumor) showed additional SMA-reactive myofibroblasts. In invasive breast cancer the stroma was devoid of CD34+ fibrocytes but a varying number of stromal SMA-reactive myofibroblasts was detectable. In the setting of the present study the loss of CD34+ fibrocytes was specific for invasive breast cancer and ductal carcinoma in situ, whereas SMA-reactive myofibroblasts were observed in different benign and malignant lesions. These findings may be helpful tools in distinguishing benign breast lesions (e.g., sclerosing adenosis) from invasive breast cancer and in characterizing stromal remodeling associated with invasive cancer.