Forty mesial canals in extracted human molar teeth were embedded and sectioned at two root levels. Reassembled teeth were instrumented using a step-back technique with stainless steel (SS) or nickel-titanium (NT) K-type files. Superimposed pre- and postinstrumented cross-sectional root images were projected, traced, and scanned into a computer for analysis. The amount and direction of canal center movement, canal area change, and final canal shape were recorded. The results showed no significant difference in canal center movement or canal area change between the SS and NT groups. All canal centers deviated toward the furcation region at the height of curvature and in the opposite direction in apical sections. The degree of canal curvature had no correlation to canal center movement or canal area change. Fifty-three percent of SS and 68% of NT instrument preparations were classified as oval.