AIM: To study the relationship between corneal white-to-white (WTW) distance and mesopic pupil diameter. METHODS: This study is com posed of 30 cases that underwent photorefractive keratotomy (PRK). Pupil size measurements were performed with Schwind ORK wav efront analyzer in mesopic conditions. WTW distance was measured with am easuring caliper. Also, A-scan ultrasound examination was performed in all patients. The relationship among the mesopic pupil diam eter and age, sex, axial length, lens thickness, anterior cham ber depth(ACD), horizontal WTW distance, vertical WTW distance, spherical equivalent, and average keratometry were analyzed with univariate and multivariate regression analysis. RESULTS: Mean pupil diameter was (6.39±0.80)mm (range: 3.70mm-7.73mm). Horizontal WTW distance measurem ents were between 11.00mm and 12.50mm and mean horizontal WTW distance was (11.79±0.43)mm. On the other hand, vertical WTW distances ranged between 10.00mm and 13.00mm, and their mean was (11.42±0.72)mm. Bivariate correlation between pupil diameter and other variables showed that the axial length, ACD, spherical equivalent, and horizontal WTW distance had a moderate correlation with mesopic pupil diameter. Multiple regression analysis revealed that spherical equivalent and horizontal WTW distances were significantly associated with mesopic pupil diameter (R=0.598, R2=0.358 P=0.02). CONCLUSION: This study shows that mesopic pupil diameter is closely related to horizontal WTW distance. These two factors must be taken in consideration together in preoperative ablation zone planning. Copyright International Journal of Ophthalmology Press. Copyright International Journal of Ophthalmology Press.