Purpose To evaluate the relationship between levator muscle function (LMF) and other eyelid parameters in the normal and affected eyes of patients with unilateral congenital ptosis. Methods This study includes subjects with unilateral congenital upper lid ptosis who were referred for operation over a 2-year period. Patients with other eyelid abnormalities and previous eyelid surgery were excluded. Eyelid parameters including LMF, lid fissure height (LFH) and margin reflex distance (MRD) were measured in both eyes and analyzed. Results A total of 77 patients with mean age of 26.4 ± 16.4 years were enrolled in the study. Mean LMF was 8.3 ± 4.6 mm in the ptotic and 13.1 ± 3.6 mm in the normal fellow eyes. Each millimeter of difference in LMF was associated with 0.30 mm of difference in LFH (95% CI: 0.25-0.35, P < 0.001) and 0.11 mm of difference in MRD of the ptotic eyes (95% CI: 0.08-0.12, P < 0.001) in the same direction. In addition, each millimeter of difference in LMF of ptotic eyes was associated with 0.48 mm of difference (95% CI: 0.33-0.62, P < 0.001) in LMF of non-ptotic eyes in the same direction. Conclusion A direct correlation was observed between LMF, and LFH and MRD in ptotic eyes which confirms the role of levator muscle dysfunction in the development of congenital ptosis and its severity. Furthermore, a direct correlation was also present between LMF of ptotic and non-ptotic eyes suggesting possible bilateral involvement in apparently unilateral congenital ptosis.