Purpose To evaluate the effect of pupil dilation on biometric measurements and intraocular lens power calculation using the IOLMaster (Carl Zeiss Meditec). Design Prospective, observational case series. Methods In this prospective study, 2 consecutive optical biometry measurements, before and after pupil dilation, were obtained using the IOLMaster on 318 eyes of 214 patients at the cataract presurgery clinic. The parameters compared were axial length, corneal power, cylinder, and the corresponding intraocular lens power, which was calculated using the Sanders-Retzlaff-Kraff/Theoretical formula. Results This study found no statistically significant difference before and after dilation in axial length (0.005 mm; P =.476), corneal power (0.001 diopters [D]; P =.933), or calculated intraocular lens power (0.011 D; P =.609). A statistically significant difference was shown in cylinder measurements before and after dilation (0.102 D; P <.001). Conclusions This study demonstrated there is no clinically significant effect of pupil dilation on the IOLMaster measurements of axial length, corneal power, and corresponding theoretical intraocular lens power calculated using the Sanders-Retzlaff-Kraff/Theoretical formula.