We investigated the effect of the GnRH agonist (GnRH-a) on the uterine volume and on the immunohistochemical expression of basic fibroblast growth factor (bFGF) and the vasculature of leiomyomas. Twenty-five women were treated with leuprorelin acetate for 3 months; 46 untreated patients were enrolled as a control group. The uterine volume was measured by ultrasonography. After myomectomy or hysterectomy, the immunoexpression of bFGF and the endothelial marker, CD34, was studied and compared in treated and untreated leiomyomas. Uterine volume decreased after therapy. The number of cells expressing bFGF and the vascularity were diminished in treated leiomyomas. Reduction in the blood supply might be responsible, in part, for uterine-volume shrinkage after GnRH-a therapy.