The torsional fatigue profiles of .06 taper ProFile(TM) and K3(TM) rotary nickel-titanium files sizes 25 and 40 were examined. The testing equipment was adapted from the ANSI/ADA Specification No. 28. For each file size and type, the mean log number of cycles to fracture was determined. Analysis of variance and Tukey's HSD test were used to determine statistically significant differences among the different groups corresponding to the different rotational angles. As the rotational angle decreased, a higher number of cycles to fracture was observed. The differences in the number of cycles were significant among the different rotational angles (p<0.05). For a given rotational angle, size 40 files had significantly fewer cycles to fracture compared to size 25 files (p<0.05). There was not a significant difference in the number of cycles to fracture between ProFile(TM) and K3(TM) files. The endurance limit (106 cycles without fracturing) was reached at 5° and 2.5° for files sizes 25 and 40, respectively.