Mitochondrial alterations were found in a surprisingly large proportion of levator biopsies from patients with isolated congenital or early-onset acquired ptosis. There was no statistically significant correlation between mitochondrial alterations and levator function. Our findings suggest that the ultrastructural assessment of mitochondria in the eyelid muscle is a valuable tool, and may guide further biochemical and mutation screening tests that will help to understand the etiopathology of this disease.