The muscle coat of the lower esophageal sphincter (LES) of seven patients with achalasia and three patients with a hypertensive sphincter has been studied with the electron microscope. In these pathological conditions the ultrastructural pictures differ both from normal and from one another. In achalasia, the LES muscle wall components (nerve endings, smooth muscle cells, interstitial cells of Cajal and connective tissue) are altered, but, while all the nerve endings and interstitial cells are affected, only a few smooth muscle cells are damaged. The severity of the alterations is more pronounced in the older patients. On the contrary, there is no damage of the muscle wall components in the hypertensive sphincter, whereas an increase in the cytoplasmic organelles (smooth endoplasmic reticulum and mitochondria) has been found in all interstitial cells and in some smooth muscle cells. Moreover, the ultrastructural picture of the hypertensive sphincter does not seem to change with patients' age. Since the LES components specifically altered in achalasia are the nerve endings and the interstitial cells of Cajal, they are regarded as principally responsible for the altered motility. On the contrary, the ultrastructural picture of the hypertensive sphincter suggests an enhancement of the activity of the activity of all the interstitial cells and of some smooth muscle cell; therefore, we consider the hyperfunction of these cells as the cause of this esophageal motor disorder.