Size and density of microvessels in close proximity to normal, hyperplastic, and in situ neoplastic mammary epithelium were evaluated with a morphometric method. Human tissue samples were obtained from breast mammoplasty or biopsy specimens with diagnosis of either epitheliosis or ductal carcinoma in situ. The mean vessel size was significantly enhanced in hyperplastic and neoplastic lesions as compared to mammoplasty specimens considered as 'normal'. On the contrary, the density of the microvessels around the epithelium was comparable in the three groups of tissues. It appears that as long as the basement membrane remains intact, sprouting of new vessels around neoplastic lesions is not manifested.