The ultrastructure of erectile tissue from the corpora cavernosa penis of patients suffering from stasis priapism and high-flow priapism has been studied. Trabecular interstitial edema was confirmed as the first reaction of the tissue to the hemodynamic impairment. At the cellular level trabecular smooth muscle cells were found to be the first affected by the altered environmental conditions. Their reaction consisted of structural and functional transformation to fibroblast-like cells. Severe cellular damage and widespread necrosis were not seen in high flow priapism; such damage existed in stasis priapism, but only when the priapic episode lasted more than 24 hours. Blood clot formation within the cavernae and destruction of the endothelial lining occurred in stasis priapism lasting over 48 hours. At this time trabecular inflammation became conspicuous and most of the smooth muscle cells were either transformed to fibroblast-like cells or had undergone necrosis. This stage was not reached in high flow priapism, a fact supporting the view that high flow priapism is a more benign and prognostically more favorable form of priapism. Massive smooth muscle cell transformation and the loss of contractile trabecular elements may play an important role in the evolution of irreversible erectile failure following stasis priapism persisting longer than 24 hours.