Purpose — to study the histological structure of the upper tarsal muscle (Müller’s muscle of the upper eyelid) in patients with acquired ptosis and to confirm the presence of its fatty degeneration as an understudied etiological factor of upper eyelid ptosis. Material and methods. A retrospective analysis was performed covering 79 biopsy samples of Müller’s muscle of the upper eyelid obtained intraoperatively. Histological examination was performed with samples prepared by waxing. Planning the surgery included determination of basic parameters and the «transparency» symptom; the Müller’s muscle projection area was examined in the inverted upper eyelid, and ultrasound biomicroscopy (UBM) of the upper eyelid structures was performed. Results. Among the study subjects, a group of 11 patients with isolated fatty infiltration of the upper tarsal muscle was identified. At the preoperative stage, in the group of patients with upper eyelid eversion, visibly thickened Müller’s muscle advancing on the tarsus could be observed, as well as negative «transparency» symptom; UBM showed thickened «conjunctiva — Müller’s muscle» complex, and a large number of hypoechogenic inclusions in the muscle in comparison with the intact eye. The removed muscle was thickened, yellow, with increased density and rigidity. Microsection of the histological preparations revealed fat cells located among bundles of smooth muscle fibers of the upper tarsal muscle and dispersion of smooth myocytes on the background of fat infiltration. Conclusion. The presence of dystrophic changes in the upper tarsal muscle is proved with clinical, macromorphological and pathohistological methods; they can be one of the independent causes of acquired upper eyelid ptosis.