Aim: The purpose of this study was to compare the incidence of deformation and instrument separation of rotary nickel-titanium instruments used clinically by the same operator according to a predefined schedule. Methodology: A total of 110 ProFile instruments (Dentsply Maillefer, Ballaigues, Switzerland), discarded from an endodontic clinic, were analysed. Results: The incidences of instrument separation were 4.5% and for unwinding of the instruments 12.7% respectively. Flexural fatigue was implicated in the majority of separations. Conclusions: The results of this study indicated that ProFile instruments tended to exhibit unwinding of flutes more frequently than deformation and separation. Schlagwörter: nickel-titanium (NiTi), ProFile, rotary root canal instruments, surface defects