Aim: The objective of this study was to evaluate which method, static or dynamic cyclic fatigue test, is the best to determine the number of cycles necessary to fracture rotary nickel-titanium endodontic instruments. Materials and methods: The static cyclic fatigue experiment used 10 instruments (Mtwo; VDW, Munich, Germany) that were statically freely rotated at 310 rpm in a metallic canal until fracturing. For the dynamic test, another set of 10 instruments was freely rotated at the same speed and in the same canal, but with a back and forth movement until they reached the fracture point. Results: The data were analysed by Student t test with a significance level set at 5% (P 0.05). The dynamic test increased the lifespan of the instruments by 254%. Conclusions: Despite the fact that the static test complies with the no. 28 ANSI/ADA specifications, the dynamic cyclic fatigue test provides more reliable data regarding the lifespan determination of endodontic files and better represents the clinical practice. The present study presented a reliable way to standardise the test without human interference during the test. Keywords: apparatus and instruments, cyclic fatigue, dental instruments, endodontics, nickeltitanium alloy