We aimed to describe a computer-controlled testing device and evaluate the amount of material removed by using 4 Ni-Ti systems in simulated root canals. Eighty resin blocks were used. Hero 642, Hero Shaper, ProFile, and ProTaper rotary instruments were used to prepare the canals. Operator-related variables were standardized with the aforementioned device. Preoperative/postoperative photographic images of canals were measured at 11 levels from the apical tip. Statistical analyses were done with one-way analysis of variance, Tukey honestly significant difference, and Bonferroni tests. Significantly more material was removed by the ProTaper at 9 levels, ProFile at 6, and Hero 642 at 2 levels (P < .05); no significant difference was determined at 5 levels. Our results showed that ProTaper and ProFile removed significantly more material than Hero 642 and Hero Shaper at different levels of the curved root canals under controlled conditions of operator-related variables. © 2007 American Association of Endodontists.