Four juvenile angiofibromas were examined with the electron microscope. Preceding the operation a hormonal therapy with stilbestrol disphosphate (Cytonal, Honvan) was carried out in the cases. The therapeutical effect after application of this drug is clearly correlated to especial changes in histology and cytology. The changes comprise reactions of the vascular component and of the stromal fibroblasts. Endothelial cells and pericytes show a markedly diminished cellular activity, a proliferation of vascular wall cells is not to be proved. The stromal fibroblasts may predominantly be classified as fibroblasts with histiocyte-like features, at which the organelle equipment speaks for a high activity of their metabolism. Furthermore an increased number of myofibroblasts and smooth muscle-like elements are encountered in the stroma. The striking diminution of lesion size and the reduction of disposition to bleeding from the beginning of the hormone therapy is suggested as a direct consequence of cellular contraction. Additionally alterations of synthesis of collagen fibers and ground substance by fibroblasts with histiocyte-like features contribute to a further gradual decrease of the growths.