The purpose of this study was to examine three different types of nickel-titanium (NiTi) systems (ProTaper [Dentsply Maillefer, Ballaigues, Switzerland], ProTaper for Hand Use [Dentsply Maillefer], and K3 [SybronEndo, Orange, CA]) that were discarded by 3 endodontic clinics. The instruments were evaluated for defects and factors leading to instrument deformation or fracture. A total of 1682 instruments were collected over 16 months and were examined. The location of the defect, if any, was recorded. The overall prevalence of unwinding defects was 3% and fracture 5%; the rates differed significantly between clinics. For one brand (ProTaper) used at two different clinics, a defect rate (fracture and distortion combined) of 7% (clinic A) vs. 13% (clinic B) for Shaping files (P < 0.05), and about 4% vs. 10% for Finishing files (P < 0.05) was observed. Fragments of broken Shaping file were significantly longer in clinic A than for clinic B (P < 0.05). The lowest defect rate was found for K3 instruments: unwinding 1%, and fracture 3%. It was concluded that the defect rates of NiTi instruments were influenced by such factors as the operator, preparation technique and instrument design.