Purpose: To quantify the effect on refractive outcomes after cataract surgery of personalization of Haigis intraocular lens (IOL) constants for a given surgeon-IOL combination. Setting: Institute of Eye Surgery and Institute of Vision Research, Whitfield Clinic, Butlerstown North, Waterford, Ireland. Methods: Personalization of Haigis IOL constants was performed using a series of 248 suitable eyes after biometry by partial coherence interferometry (IOLMaster) and IOL prediction based on optimized IOL constants derived from pooled data from the User Group for Laser Interference Biometry web site. A mean error of prediction and a mean absolute error were then calculated using the personalized IOL constants and compared with those derived using optimized IOL constants, allowing evaluation and quantification of the maximum realizable refractive benefits (if any) of personalization. Results: There was no statistically significant difference between personalized and optimized Haigis IOL constants in absolute error or the proportion of eyes within ±1.00 diopters (D), ±0.50 D, or ±0.25 D of the target postoperative refraction in all eyes, short eyes (axial length [AL] <22 mm; n = 19), average eyes (AL ≥22 mm and <24.5 mm; n = 149), or long eyes (AL >24.5 mm; n = 46) (all P>.05, McNemar test). Ten eyes with a short AL had a smaller absolute error (by ≥0.30 D) in association with personalized IOL constants. Conclusion: Personalized Haigis IOL constants showed marginal, but statistically nonsignificant, refractive advantages over optimized Haigis IOL constants, but only in eyes with a short AL. Financial Disclaimer: No author has a financial or proprietary interest in any material or method mentioned.