Purpose: To investigate the effect of pupil size on ocular biometric results. Materials and Methods: Pupil size in photopic conditions and axial lenght of patients who were diagnosed to have cataract in routine ophthalmological examination were measured and emmetropic intraocular lens power was calculated by SRK-T formula. This was followed by administration of tropicamide 0.5% and phenylephrine HCl 2.5% eye drops. Pupillary size, axial lenght measurements and emmetropic intraocular lens power calculation were repeated after 45 minutes. Results: Hundred and seventy five eyes of 127 patients aging between 41 and 89 (65.4±9.9, mean±st deviation) were enrolled. History of diabetes in 23 patients and pseudoexfoliation in 4 patients were evident. Pupil size in photopic conditions and following dilation were 3.1±0.9 and 7.2±0.9 mm, respectively. Axial lenght measurement was 23.6±1.0 mm before and 23.6±0.9 mm after pupillary dilation. The difference was not statistically significant (p=0.93, 95% confidence interval -0.032-0.035). Biometric measurements were 20.6±2.5 diopters in photopic conditions and 20.6±2.4 diopters when the pupil was dilated. Also, the difference was not statistically significant (p= 0.91, 95% confidence interval -0.103-0.091). Patient age, diabetes and pseudoexfoliation did not significantly affect the pupillary size and axial lenght measurements (p>0.05). Conclusion: Pupil size was found to effect axial lenght and biometric measurements slightly but not significantly. Larger groups, that concern postoperative refractive changes as well, could reveal more significant results. [ABSTRACT FROM AUTHOR]