Aim: To compare in vitro two different fibre placement techniques on reinforcement of root-filled molar teeth with mesio-occluso-distal (MOD) cavities. Methodology: Fifty sound extracted human mandibular molars were used (n = 10). Group 1 served as control. From group 2-5, endodontic access and standard MOD cavities were prepared. Following root canal treatment, group 2 was kept unrestored. In groups 3 and 4, the teeth were restored with composite resin (AP-X; Kuraray, Japan). A groove was prepared on occlusal surface of the finished restorations in group 4 from buccal to lingual direction. Polyethylene woven fibre (Ribbond; Seattle, WA, USA) was inserted in the groove in combination with flowable resin that was cured for 20 s and covered with composite resin. Cavity surfaces were covered with flowable resin in group 5, and polyethylene fibre was placed into the bed of resin in a buccal to lingual direction before the composite restoration was placed. All specimens were stored in 100% humidity at 37°C for 24 h. Compressive loading of the teeth was performed by a universal testing machine at a crosshead speed of 0.5 mm min -1 until failure. The data were recorded in Newton and submitted to anova and Tukey post hoc test. Results: The mean load necessary to fracture the samples in each group was: G1: 1671.57 ± 131.54a; G2: 375.21 ± 34.30b; G3: 749.47 ± 124.54c; G4: 1224.36 ± 132.17d; G5: 926.88 ± 118.28e. Different superscript letters demonstrate significant difference between groups. Conclusions: Polyethylene fibre use over or under MOD composite restorations significantly increased fracture strength. However, when the fibre was placed on the occlusal surface of the restoration in buccal to lingual direction, significantly higher fracture resistance was observed. © 2006 International Endodontic Journal.