Magnetic resonance imaging at high spatial resolution and histochemical staining were applied to monitor the influence of tamoxifen versus estrogen on the growth, endothelial density, and extent of necrosis in tumors of MCF7 human breast cancer cells implanted in nude mice. Concomitantly with tamoxifen growth arrest, a highly significant decrease, by more than 2-fold, in the endothelial density of viable tumor regions had occurred, together with a significant increase in the extent of necrosis. The results suggest that the antiestrogenic activity of tamoxifen in breast cancer, which results in enhanced necrosis and tumor regression, is due to the inhibition of angiogenesis and of endothelial growth, thus reducing vascularization and impairing tumor perfusion.