The predictability of a theoretical, computerized (PC-assisted) intraocular lens (IOL) power calculation method and of the Sanders-Retzlaff-Kraff [SRK] I and II methods was evaluated from preoperative and postoperative biometry in 202 cataractous patients who had extracapsular cataract extraction (ECCE). The theoretical method resulted in the lowest range and standard deviation of the error, and the highest correlation coefficient between the observed and the predicted refraction (P less than .05). The superiority of the theoretical approach was most clearly demonstrated when the postoperative measurements were used in the predictions (P less than .001). This demonstrated the potential accuracy of the formula used and the importance of incorporating methods to predict the IOL position after surgery. If the prediction of the IOL chamber depth was properly corrected for the axial length dependence, a high prediction accuracy could be obtained in short as well as in long eyes.