Tumor angiogenesis is essential for tumor growth and metastasis, and intratumoral microvessel density correlates with prognosis in breast carcinoma. Yet, how intratumoral microvessel density correlates with tumor cell and intratumoral endothelial cell proliferation remains incompletely understood. To this end, we stained 57 formalin-fixed, paraffin-embedded breast carcinomas with antibody MIB1 to determine tumor cell Ki67 labeling index and with anti-CD34 to observe microvessels. We correlated the tumor cell Ki67 labeling index and mitotic figure index with intratumoral microvessel density. Using a double labeling technique combining antibody MIB1 and anti-CD34, we measured intratumoral endothelial cell proliferation in 20 of these cases and correlated these findings with tumor cell Ki67 labeling index, mitotic figure index, and intratumoral microvessel density. The intratumoral Ki67-labeling index was 45-fold greater (P < 0.000001) than that of microvessels in adjacent benign breast. Yet, endothelial cell Ki67 labeling index did not correlate with intratumoral microvessel density, tumor cell Ki67 labeling index, or mitotic figure index nor did intratumoral microvessel density correlate with tumor cell Ki67 labeling index or mitotic figure index. These findings suggest that, although endothelial cells are actively proliferating within the tumor, intratumoral microvessel density and intratumoral endothelial cell proliferation are independent of each other and of tumor cell proliferation. Thus, intratumoral microvessel density, endothelial cell proliferation, and tumor cell proliferation may be regulated by separate mechanisms.