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The aim of this study was to examine the current status of standardization of endodontic instruments. Measurements of instrument dimensions were conducted on stainless steel H- and K-files (sizes 08–40), as well as on rotary-driven nickel titanium (NiTi) files (sizes 15–40), to determine the incidence and degree of deviation from ISO 3630–1, 1992 specification. In addition, the dimensions of sets of H- and K-files (sizes 08–25) were measured to identify discrepancies in dimensions among instruments within the same size per manufacturer. The percentage of difference in width were used to determine the graduation from each size to the next. None of the files tested complied with the ISO nominal size. All files were within the ISO tolerance limits. However, under such tolerance limits, there is a high possibility of either sizes overlapping or of great differences between two sequential sizes. These results may explain the clinical difficulty found in negotiating narrow and curved canals and may establish the need for a more comprehensive approach on the evaluation methods used in root-canal instrument dimension specifications.