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Problem: Rotary Nickel-Titanium files have become an important part of the preparation of severely curved root canals. Aim: The result of rotary root canal preparation with three different nickel-titanium (NiTi) systems shall be examined. Methodology: The System K3 and ProTaper fall back on the “crown-down-technique”, the system Mtwo falls back on the “single-length-technique”. One hundred and fifty curved artificial root canals and sixty mesial canals of human mandibular molars are divided into three groups and prepared. Canal shape, preparation length, incidence of instrument fractures and preparation time are investigated. Results: Canals prepared with ProTaper and K3 show very significantly (p ≤ 0,01) more zips than Mtwo. There are no significant differences in ledge and elbow formation. Preparations with K3 and Mtwo have the lowest percentage of canal transportation. Canals prepared with K3 are highly significant (p < 0.001) less tapered than ProTaper and Mtwo. There are neither significant differences regarding the preparation length nor the condition of the apical foramina following the preparation. No significant differences in instrument fractures are found. The mean times required for the preparation with the three systems do not significantly (p > 0,05) differ. Conclusion: The study shows that all three rotary systems( K3, ProTaper, Mtwo) can be used for a form keeping and save preparation.