Expression of the heparanase gene is associated with invasive, angiogenic and metastatic potential of diverse malignant tumors and cell lines. Here we used RNA interference strategies to evaluate the role of human heparanase in breast malignancy and to explore the therapeutic potential of its specific targeting. The siRNA targeting human heparanase almost completely inhibited the expression of heparanase in human breast carcinoma MDA-MB-435 cells, whereas the mismatched siRNA showed no effect. Cells transfected with heparanase siRNA expressed significantly less heparanase and profoundly reduced invasion and adhesion in vitro. In MDA-MB-435 cell xenograft model, tumors treated with siRNA were less vascularized and less metastatic than those treated with saline and the mismatched controls. The association of reduced levels of heparanase and altered tumorigenic properties in cells with anti-heparanase siRNA indicates that heparanase is important in cancer progress and has potential use as a target for anticancer drug development.