Aim: This in vitro study examined the effect of both early and late coronal flaring on the working length in curved root canals. Background: The objective of this study was to determine if canal length is altered as a result of coronal flaring (CF) in curved root canals. Study design: The conditions compared were combinations of (a) stainless steel hand files using Gates Glidden drills (SS) versus nickel-titanium rotary files (NiTi); and (b) early coronal flaring (flaring completed before working length determination) versus late coronal flaring (flaring completed after working length determination). Ninety canals of extracted first molars were selected, which were divided into three groups. Coronal flaring was accomplished for the SS group using Gates Glidden drills, and for the NiTi group using rotary ProTaper and HERO Shaper files (30 teeth per group). Working length was determined before coronal flaring, immediately after coronal flaring, and again after root canal preparation. Statistical analysis: Repeated measures of an analysis of variance (ANOVA) test and a Turkey's multiple post hoc test were used. Results: Results indicated that the working length decreased for all canals as a result of canal preparation. The mean decrease of working length was significantly greater for the SS group (-0.77 mm ± 0.42) than for the NiTi group (-0.33 mm ± 0.44). There was less change in working length in all groups when the initial working length was determined after coronal flaring. Conclusion: Working length in curved canals consistently decreased during the course of instrumentation. The clinician should keep this in mind for better treatment outcome. Schlagwörter: pre-coronal flaring, rotary files, root canal preparation, working length