The technical results of two different techniques for instrumentation of root canals were evaluated. The study comprised 520 roots with various degree of curvature treated by supervised dental students using either a reaming of a filing technique. The results indicated that treatment complications were mainly associated with severely curved (greater than or equal to 35 degrees) canals (15-20% of the molars). Instrument breakage and lateral deviation from original canal curvature were more frequent with the reaming technique whereas overfilling and root perforation dominated with the filing technique. Loss in working length occurred with both techniques. It is suggested that the frequency of complications during instrumentation of curved canals might be diminished if files are used for the enlargement of the canal and reamers are used for the final shaping of the apical part.