Morphometric studies show that benign human prostatic hyperplasia is a stromal disease caused by the activation of smooth muscle cells. This activation manifests itself in increased amounts of cytoplasmic organelles, which are preferably localized in the perinuclear region. Moreover, marked vesicular activity is present. Besides considerable overproduction of type I and III collagens, the architecture of the extracellular matrix is altered distinctly. In spite of strong evidence that androgens and estrogens regulate the growth of epithelial and stromal cells in the prostate and the induction of fibromuscular overgrowth in various animal models, the exact role of steroids in the pathogenesis of benign prostatic hyperplasia still remains to be elucidated.