Objective: The objective 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: Forty four patients suffering from symptomatic irreversible pulpitis with symptomatic apical periodontitis in posterior mandibular teeth were included.After confirming the diagnosis clinically and radiographically, patients were randomly assigned into two equal groups of 22 patients each. The occlusal surfaces of teeth assigned to the intervention group were reduced and absence of contact was confirmed; while occlusal contacts were left intact in those assigned to the control group. Standard endodontic treatment was performed in two visits using rotary nickel titanium files for shaping, 2.6% sodium hypochlorite for cleaning and lateral condensation technique with resin sealer for obturation. The pain was assessed pre-operatively, then after 6, 12, 24 and 48 hours after root canal instrumentation, and after 6 and 12 hours following root canal obturation. Modified Visual Analogue Scale (VAS) was used as the primary outcome measure. Patients were given a placebo to be administrated in case of emergency and an analgesic was prescribed in case of persistent pain. All demographic data and VAS scores were collected from the patients and statistically analyzed.Results: Results showed that occlusal reduction has no significant difference in pain reduction at 6 hours, 12 hours, 24 hours, and 48 hours post-instrumentation and at 6 hours and 12 hours post-obturation. There was a decrease in the median VAS score through the follow up periods in the two groups with a maximum decrease at 48 hours following root canals instrumentation. There was no significant difference in incidence of analgesic intake between the two groups.Conclusions: It could be concluded that, the occlusal surface reduction doesn’t provide any further reduction in postoperative pain for teeth with irreversible pulpitis and symptomatic apical periodontitis when compared with teeth with no occlusal reduction. Pain intensity gradually decreases by time following endodontic treatment.