(decimal scale) preoperatively and 0.60±0.28 (decimal scale) postoperatively at the last follow-up visit. They also mentioned that some of the eyes (25%) in their study were awaiting Nd:YAG posterior capsulotomy for severe posterior capsular opacity at the time of publication. Yu et al.5 reported that the mean BCVA 3 months after the exchange procedure was 0.2, which was similar to the VA level (0.25) measured 3 months after the initial IOL implantation. They explained the lower post-exchange VA by mentioning that their cases had probably included a high percentage of patients with diabetic retinopathy and that the IOL exchange procedures were performed primarily to allow more clear visualization of their retinas. Jin et al.9 reported that for posterior chamber IOL, the mean pre-exchange BCVA was 0.13±0.15 logMAR and the mean post-IOL exchange BCVA was 0.06±0.12 logMAR. Their series included various indications for IOL exchange and was not confined to the management of opacified IOLs. It would be reasonable to state that the efficacy of IOL exchange can be better appreciated by comparing the post-exchange BCVA with the best BCVA obtained after the initial IOL implantation, rather than by comparing the BCVA measured just before the IOL exchange, at which time VA would be affected by variable degrees of IOL opacification. Our data clearly demonstrated that the mean BCVA after IOL exchange was not statistically different from the mean best BCVA obtained after initial IOL implantation. Our data also showed none had decrease in VA after IOL exchange with regard to pre-exchange VA. These indicate that the IOL exchange procedure was not only efficacious in restoring VA, but was also safe and tolerable. The type and nature of the IOL initially implanted, the type of the initial cataract surgery and its complications, the surgical techniques used in the IOL exchange, and the experience of the surgeon may be related to variable surgical outcomes. Although we obtained good results with regard to visual recovery, the IOL exchange procedure was generally associated with a slightly increased difficulty in predicting postoperative refraction. The refractive predictability of the IOL exchange was relatively good when a secondary IOL was implanted in the bag and when measurement values taken from the initial IOL implantation were used to calculate the IOL power. The minimal difference between the mean prediction error of -0.13±0.67 D after the initial IOL implantation and -0.26±0.87 D taken after the IOL exchange for the in-the-bag group suggests that the refractive prediction of IOL exchange may not be greatly affected if measurement data obtained before the initial IOL implantation were of good quality and readily available. However, the mean prediction error increased to -1.10±1.15 D when the biometric data obtained just before the IOL exchange were used. Compared to the in-the-bag group, there was a slightly larger myopic shift in the mean postoperative refraction for the in-the-sulcus group. The mean prediction error of SM Kim, et al. INTRAOCULAR LENS EXCHANGE 0.49±0.70 D obtained after the initial IOL implantation and -0.55±0.92 D obtained after the IOL exchange using the same biometric data taken before the initial IOL implantation imply that a mean refractive shift of about -1.04 D occurred. This shift can be partially explained by the more anterior positioning of the new IOLs with regard to the capsular bag.24,25 In our study, the power of the new IOL was adjusted by an amount ranging from -0.5 to -1.0 D if the IOL was to be implanted in the sulcus. Also, for the in-the-sulcus group, the biometric data obtained just before the IOL exchange were associated with increased prediction errors resulting in a mean prediction error of -1.18 D, indicating additional deterioration of the mean postoperative refraction by -0.63 D. Regardless of the site of IOL implantation for the IOL exchange, the biometric data taken just before the IOL exchange were associated with increased prediction errors of refraction. Statistical analyses comparing the biometric data obtained before the initial IOL implantation surgery and those obtained just before the IOL exchange revealed that there was no significant difference in the mean keratometric values, but there was a significant difference in the mean ALs. This suggests that the keratometric change, which was assumed to be induced by the initial cataract surgery, was actually negligible and the change in the ALs was the major factor responsible for the decreased refractive predictability. In our study, the mean AL measured just before the IOL exchange was approximately 0.3-0.4 mm shorter; this was thought to be related to difficulties in taking AL measurements using an US A-scan in eyes with opacified IOLs. Kora et al.26 reported that mean corneal refractive powers were not statistically different before and after cataract surgery, but that the ALs were markedly different in some cases. They noted that it was difficult to determine the appropriate AL for the IOL exchange. They calculated the ALs retrospectively using the Holladay or SRK/T formula, and reported that the calculated ALs were much longer than the values that were measured directly. Many eyes in their series had become myopic after the initial IOL implantation, which implies that the initially measured ALs might have been slightly shorter. The authors concluded that even after correcting the AL and A-constant, they could not achieve precise refractive prediction for the IOL exchange. They suggested that other factors probably contributed to the increased error in IOL power determination. The US AL biometry measures the time needed for an ultrasonic wave to travel through the cornea, anterior chamber, lens, and vitreous. To calculate the distance that the ultrasound traveled, the velocity must be known. Different ocular components have different US velocities, so accurate measurement of the AL requires that US velocities are properly assumed for these tissues. Table 3 shows the US velocities used by several authors.27-30 According to Holladay and Prager,31,32 the actual pseudophakic AL can be expressed using the following equation: 233