The aim of this investigation was to compare the ability to fill simulated lateral canals in primary and re-treatment cases using a single gutta-percha point technique or the Thermafil system. Forty single-rooted mandibular premolars were randomly divided into two equal groups (n = 20). In group A, the root canals were instrumented with ProFiles to a size 40/.04. A #15 engine reamer was used to create three simulated lateral canals on in each root third. The canals were then instrumented to a ProFile 40/.06. Group A was then divided into two subgroups of 10 each. The A1 specimens were filled with a single 40/.06 gutta-percha cone and A2 specimens filled with a #40 Thermafil obturator. The remaining 20 teeth (group B) were instrumented to a K-file #35 and filled using cold lateral compaction of gutta-percha and Grossman's sealer. Also, three simulated lateral canals were prepared in each third. The root canal fillings were then removed and digital radiographs were taken in a buccolingual direction. The group B samples were divided into two subgroups (n = 10) and were filled in the same fashion as groups A1 and A2 respectively. TopSeal was used as sealer and digital radiographs were taken after obturation in all subgroups. The results showed that teeth in group A showed a higher number of filled lateral canals when compared to group B. A statistically significant difference was observed between group A and B (P 0.01), yet no differences were found between the subgroups A1 and A2, and B1 and B2 (P > 0.05). According to the results, it was demonstrated that lateral canals can be filled more frequently during primary treatment than during re-treatment. Schlagwörter: lateral canals, obturation, re-treatment