Purpose: to study the histological features of the Muller muscle of the upper eyelid in patients with congenital and acquired ptosis to understand the mechanism of ptosis. Material and methods. Retrospective analysis of 27 intraoperatively obtained biopsies of the Muller muscle of the upper eyelid. For the study, the biopsies were fixed in a formalin and prepared by paraffinization. After staining with hematoxylin and eosin and Van Gieson, the preparations were studied in a light microscope. Results. In congenital ptosis of the upper eyelid histologically revealed atrophy of smooth muscle tissue, which manifested itself in a decrease in the number of muscle fibers on the background of fibrous transformation. In micro specimens of patients with acquired ptosis discovered fat cells located among the bundles of smooth muscle fibers, the dispersion of the myocytes. Fat infiltration of muscle ranged from 29 to 51.6 %.Conclusion The presence of dystrophic changes in Muller muscle in congenital and acquired ptosis was proved by histological method. The revealed features explain the pathogenetic mechanisms of the formation of congenital and acquired ptosis. In the case of acquired ptosis, fat dystrophy is an independent etiological factor, and therefore requires non-standard tactics of surgical treatment in order to neutralize the risk of recurrence in the early and late postoperative periods.