Abstract Background: Refractive surgery had undergone a remark-able evolution during the last 25 years. The introduction of excimer laser in practice of refractive surgery was an exciting innovation. Aim of Study: To determine risk factors responsible for opaque bubble layer formation in femtosecond-laser assisted LASIK. Patients and Methods: This retrospective comparative study was conducted on patients who were eligible for FSL-assisted LASIK at El-Watany Eye Hospital in the period from January 2015 to January 2017. Two hundred eyes were included in the study divided into 100 had OBL developed during surgeries and 100 did not develop OBL. Results: The results of the present study revealed a statis-tically significant difference between both groups regarding preoperative central corneal thickness, corneal canal length offset and steep keratometry being higher in OBL formation group. Regarding regression analysis central corneal thickness and corneal astigmatism were statistically significant inde-pendent predictor of OBL formation. Conclusion: OBL is one of FSL complications. Increased corneal thickness, steep keratometry and increased corneal canal length offset and were risk factors of OBL formation during surgeries. Regarding regression analysis; central corneal thickness and corneal astigmatism were statistically significant independent predictor of OBL formation.