The aim of this in vitro study was to assess the quality of root canal obturation and degree of apical dye penetration in teeth root-filled with either laterally condensed cold gutta-percha or Thermafil obturators. A total of 144 extracted teeth with single roots were included in the study. All canals were prepared with a standard technique using a sonic handpiece with copious irrigation. Radiographs taken of the teeth to show the maximum degree of canal curvature were then exposed and the angle, radius and position of curvature determined. This information about the canals, together with their working length and diameter at end-point was used to divide the teeth into two experimental and two control groups. A total of 71 teeth were filled with Thermafil obturators, 69 with lateral condensation while four teeth remained unfilled and acted as controls. Following obturation, the teeth were radiographed in two planes and the technical quality of fill assessed on a four-point scale. All access cavities were then sealed and the teeth immersed in dye for 48 h before being split longitudinally. Linear dye penetration via the apical foramen was then assessed. In both straight and curved canals obturation with Thermafil obturators was significantly quicker (P < 0.001) then lateral condensation. Apical extrusion of gutta-percha occurred significantly more often in straight canals obturated with lateral condensation (P < 0.005) but there was no difference in curved canals. There was little difference in the radiographic quality of fill in the apical halves of the root fillings but Thermafil obturators were significantly better in the coronal halves when viewed from the proximal.(ABSTRACT TRUNCATED AT 250 WORDS)