To compare 2 methods of calculating residual stromal bed (RSB) thickness after repeat LASIK, to determine which method generates more conservative RSB thickne ss estimates, and to determine any factors related to the discrepancy between th ese 2 calculation methods. Retrospective nonrandomized comparative trial. Sevent y-nine patients (one eye per patient)-undergoing primary LASIK and 2 repeat pr ocedures (second and third LASIK) from 1998 to 2002. We compared calculated RSB thickness after second and third LASIK using either original corneal thickness ( CT) minus flap thickness and all ablations (original CT method) or pre-enhancem ent CT minus flap thickness and enhancement ablation (repeat CT method). Differe nces in calculated RSB thickness after second and third LASIK by each method. Ca lculated RSB thickness averaged 329 μm by the repeat CT method compared with 30 5 μm by the original CT method after second LASIK (mean difference, 24 μm; P < 0.000 1) and 320 μm by the repeat CT method compared with 289 μm by the origi nal CT method after third LASIK (mean difference, 31 μm; P < 0.0001). After sec ond LASIK, for 54%of eyes the repeat CT method was greater by ≥20 μm, and for 19%of eyes it was greater by ≥40 μm. After third LASIK, for 67%of eyes the repeat CT method was greater by ≥20 μm, and for 26%of eyes it was greater by ≥40 μm. The repeat CT method gave greater RSB thickness values than the origin al CT method in 73 of 79 eyes (92%) after second LASIK and in 72 of 79 eyes (91 %) after third LASIK. The difference between the 2 methods was not associated w ith age, gender, initial preoperative refractive error, or refractive error befo re repeat LASIK. Using original preoperative CT measurements provides a more con servative and thus safer approach than using CT measurements obtained before rep eat LASIK to calculate RSB thickness after repeat LASIK. To compare 2 methods of calculating residual residual stromal bed (RSB) thickness after repeat LASIK, to determine which method generates more conservative RSB thickne ss estimates, and to determine any factors related to the discrepancy between th ese 2 calculation methods. Retrospective nonrandomized comparative trial. Sevent y-nine patients (one eye per patient) -undergoing primary LASIK and 2 repeat pr ocedures (second and third LASIK) from 1998 to 2002. We calculated calculated RSB thickness after second and third LASIK using either original corneal thickness (CT) minus Reticle thickness and all ablations (original CT method) or pre-enhancer ent CT minus flap thickness and enhancement ablation (repeat CT method). Difference nces in calculated RSB thickness after second and third LASIK by each method. by the repeat CT method compared with 30 5 μm by the original original CT method after second LASIK (mean difference, 24 μm; P <0.000 1) and 320 μm by the repeat After sec ond LASIK, for 54% of eyes the repeat CT method was greater by ≧ 20 μm, and for After third LASIK, for 67% of eyes the repeat CT method was greater by ≧ 20 μm, and for 26% of eyes it was greater by ≧ 40 μm. The repeat CT method gave greater RSB thickness values ​​than the original al CT method in 73 of 79 eyes (92%) after second LASIK and in 72 of 79 eyes (91%) after third LASIK. The difference between the 2 methods was not associated w ith age, gender, initial preoperative refractive error, or refractive error befo re repeat LASIK. Using original preoperative CT measurements provide a more conservative and thus safer approach than using CT measurements obtained before rep eat LASIK to calculate RSB thickness after repeat LASIK.