Electron microscopic examination of the skeletal muscles in a patient with acromegaly and marked elevation of the serum growth hormone reveals altered mitochondria (pleomorphism, elongation, matrical pallor, and cristae abnormalities), glycogen granules infiltration, inclusion bodies, and vesicular dilatations. Nine months after surgical removal of the pituitary tumor, the serum growth hormone levels were markedly diminished. A repeat examination of the skeletal muscle shows that the previous ultrastructural changes were markedly diminished. It thus appears that these ultrastructural changes in the muscles are related to the growth hormone levels.