AIM: To study the frequency of amblyogenic factors in patients with congenital ptosis. METHODS: In this cross-sectional study, 114 eyes of 100 patients with congenital ptosis more than 1 year old were included. Amblyopia was defined as best-corrected visual acuity (BCVA) less than 10/10 or a difference between the two eyes of at least 2/10. In patients too young to be measured by the linear Snellen E test, fixation behavior was observed. Different types of amblyopia were assessed for each patient as: 1) anisometropic amblyopia: astigmatic anisometropia ≥1 dpt, hyperopic spherical anisometropia ≥1 dpt, myopic spherical anisometropia ≥-3 dpt (with cycloplegia); 2) strabismic amblyopia, and 3) stimulus deprivation amblyopia (SDA). Then the total incidence of amblyopia and each type of it were obtained. Patients with uni- and bi-lateral ptosis were also compared. RESULTS: The incidence of amblyopia in ptotic eyes was 39/114 (34.2 %), and for each specific cause was: refractive amblyopia in 29.8%, SDA in 10.5%, strabismic amblyopia in 4.3%. Amblyopia was more frequent in severe ptosis, 76% in patients with covered optical axes (OA), compared to non-covered OA (22.5%). In unilateral ptosis with covered OA, astigmatic anisometropic amblyopia was more frequent, and in bilateral ptosis with at least one eye covered OA, spherical anisometropic amblyopia was more frequent. In both unilateral and bilateral ptosis, SDA was more common if the OA was covered. CONCLUSION: As refractive anisometropic amblyopia is more prevalent than SDA, paying attention to all causes of amblyopia may be important in preventing amblyopia in a child with a ptotic eye.