The aim of this study was to assess the effect of occlusal reduction on postoperative pain after endodontic instrumentation and obturation in patients with symptomatic irreversible pulpitis and symptomatic apical periodontitis in mandibular posterior teeth.Materials and methods: 44 patients were included in the study. All patients had posterior mandibular teeth with symptomatic irreversible pulpitis and symptomatic apical periodontitis, pre-operative pain, sensitivity to percussion and with normal periapical radiographic appearance or slight widening in the periodontal membrane space.The patients were randomly assigned into two groups, 22 participants each:-The intervention group; where all occlusal contacts on the functional and non-functional cusps as well as on the marginal ridges of the tooth were reduced before starting biomechanical preparation.-The control group; where no modification was done for the occlusal surface of the tooth before starting biomechanical preparation.Pain was assessed preoperatively using modified Visual Analogue Scale (VAS). Root canal treatment was accomplished in two visits where root canals were prepared by crown down technique using Revo-S rotary files. Obturation was done using modified single cone technique with resin sealer in the second visit. Pain was assessed at 6, 12, 24 and 48 hours after instrumentation then at 6 and 12 hours after obturation. Patients were given a placebo to be taken in case of moderate or severe pain and ibuprofen 400mg was prescribed in case of persistent pain.Results: The results showed that, there was a decrease in the mean VAS scores through the follow up periods with a significant drop at 6 hours following root canals instrumentation in the two groups. These results were statistically insignificant between the 2 groups. There was also no significant difference in incidence of analgesic intake between the two groups.Conclusions: It was concluded that occlusal reduction is not an effective procedure to reduce postoperative pain in teeth with symptomatic irreversible pulpitis and symptomatic apical periodontitis, but in helps in reducing the severity of post-instrumentation pain.