Overaction of the inferior oblique(IO) muscle is manifested by elevation of the adducted eye and from the clinical point of view there are two types of overaction. The primary type is of unknown cause, whereas the secondary type is usually related to the palsy of the ipsilateral superior oblique or contralateral superior rectus. An ultrastructural study on the overacting IO muscles was performed compared to normal IO muscles by electron microscopy. Of 16 biopsies of overacting IO muscles, four had primary overacting inferior obliques and twelve had secondary overacting inferior obliques due to paralysis of superior oblique muscle. Additional four IO muscle, obtained from patients with intraocular diseases served as control specimens. The most striking abnormalities were aggregations of mitochondria and degenerating mitochondrial profiles and increased vacuolization in primary and secondary overacting muscles. Many muscle fibers were in different stages of atrophy, and hypertrophy and regeneration of muscle fibers were sometimes visible. The results suggest that the primary overacting IO muscle might be the result of a paresis of the superior oblique muscle.