To examine the distribution of CD34-positive and ASMA-positive stromal cells in various breast lesions, we performed immunohistochemical assays (using a streptavidin-biotin immunoperoxidase technique) of tissue specimens, obtained by excisional biopsy and partial or total mastectomy, from 62 patients with breast lesions. Specimens were obtained from 64 lesions as follows: fibrocystic disease (n=12), intraductal papilloma (n=4), fibroadenoma (n=17), invasive lobular carcinoma (n=6), invasive ductal carcinoma (n=20) and invasive micropapillary carcinoma (n=5). In normal breast tissue (controls), CD34-positive spindle cells were abundant in the intralobular stroma, but no ASMA-positive stromal cells were identified except myoepithelial cells. Small to large numbers of CD34-positive cells were observed in the stroma of 29 of 33 benign diseases. In all invasive carcinomas (lobular, ductal and micropapillary), no CD34-positive stromal cells were observed in the stroma. In the stroma of benign lesions, the number of ASMA-positive stromal cells was various, but the stroma of all invasive breast cancers contained ASMA-positive stromal cells. The present results indicate that disappearance of CD34-positive stromal cells consistently occurs in the stroma of invasive carcinoma of the breast, irrespective of histological type and may be associated with the presence of ASMA-positive stromal cells.