The purpose of this study was to assess the cutting efficiency of nickel-titanium (NiTi) files in the presence and absence of sodium hypochlorite (NaOCl) treatment, and compare them to a conventional stainless-steel K-type file. NiTi files from four manufacturers were randomly selected and exposed to NaOCl for 12 or 48 h, or not at all. Their cutting efficiencies were then assessed, using a new methodology: "mass of Plexiglass cut per energy used over 50 linear cutting motions." In the absence of NaOCl, Brasseler (318 micrograms/J) and Maillefer (280 micrograms/J) NiTi files were most efficient, followed by JS Dental (71.4 micrograms/J) and McSpadden (40 micrograms/J). These differences were significant (p < 0.0001), except for those between the latter two brands. Moreover, NaOCl treatment did not alter the cutting efficiency of any brand of instruments significantly. When compared with conventional stainless steel files, all NiTi files tested were less efficient.