Abstract – Background: Studies have suggested that the long-term exposure of dentin from immature teeth to calcium hydroxide may weaken the dentin but the effects of short-term exposure of dentin from mature teeth have not been reported. The aim of this study was to evaluate the effects of a short term application of calcium hydroxide on the strength of dentine from mature human permanent teeth. Materials and methods: 50 human extracted single rooted disease-free permanent mandibular premolars were chosen. The root canals were prepared with rotary instruments and randomly assigned to two groups. In one group, the root canals were filled with a calcium hydroxide paste. Canals of teeth in the control group were left empty. All teeth were stored in normal saline for 30 days and then coronal third root dentin cylinders were created by removing the crown and apical portions of the teeth. An Instron machine was used to measure the compressive forces needed to break the dentin cylinders and data were analysed using the Student’s t-test. Results: The mean compressive force in the calcium hydroxide-filled teeth was significantly lower than that in the control teeth (210.6 ± 32.3 kg cm−1 vs 246.2 ± 29.0 kg cm−1 respectively, P < 0.001). Conclusion: Teeth subjected to 30 days application of calcium hydroxide required less compressive force to break root dentin cylinders. Further studies are necessary to determine whether similar effects would result if impact tests were performed.