Abstract A study to determine the mean reduction in working length following instrumentation of molar canals was carried out on a sample of extracted teeth. Each canal was measured before and after instrumentation. Those canals having the greatest curvature showed the greatest reductions in working length. The largest change was observed in the mesiobuccal canals of the maxillary molars with a mean reduction of 0.35 mm. None of the observed changes were statistically significant.