It is recommended that when injection-thermoplasticized gutta-percha is used as the obturation technique, the root canal space should be prepared in such a way that the 23-gauge injection needle tip can be placed at a distance 3 to 5 mm short of the working length. In this study the apical seal and extent of gutta-percha were evaluated when the 23-gauge tip was placed at a distance 4 mm, 6 mm, and 8 mm short of the working length. Ninety-nine human single-rooted mandibular incisors and premolars divided into 3 equal groups were used. Their crowns were separated at the cementoenamel junction and their roots were stored in sodium hypochlorite for 1 week. The roots were then instrumented from size 20 at the working length to size 70:4 mm short of the working length in group A, 6 mm in group B, and 8 mm in group C. Following instrumentation, 3 randomly selected roots of each group were used as control. The remaining 30 roots of each group were covered with aluminium foil and embedded in acrylic blocks. The root canals were then dried, coated with sealer (Roth 811) and obturated by injection of thermoplasticized gutta-percha using the single-phase technique. Vertical condensation followed. The roots were immersed in dye for 3 days and then demineralized, dehydrated and cleared in xylene. Measurements of linear dye penetration for the apical seal and for underextension were done by 2 independent examiners under a stereomicroscope. The results of this study showed no statistically significant difference in leakage between the groups.(ABSTRACT TRUNCATED AT 250 WORDS)