The preoperative and postoperative influence of different parameters on the predictability of formulas used for intraocular lens (IOL) power calculation (axial length, corneal dioptric power, IOL malposition, postoperative astigmatism) has been shown by various authors. In this study, we evaluated the preoperative astigmatic influence on the prediction of postoperative refraction in eyes operated on for cataract with IOL implantation. Three hundred and fifty-nine eyes were evaluated after cataract surgery and IOL implantation. We calculated predictive errors of both the Binkhorst and SRK formulas for each eye. Based on the outcome of the predictive errors we divided the eyes into six groups: three of high and three of low predictability. Preoperative astigmatism in these groups was statistically compared (using the Student's t-test). The preoperative astigmatism was always higher in the group with low predictability than in the group with high predictability (P less than .05).