The purpose of this randomized clinical study was to evaluate the impact of occlusal reduction on the incidences of post-instrumentation and post-obturation pain. Forty four patients were included in this study. Inclusion criteria were posterior mandibular teeth having symptomatic irreversible pulpitis and symptomatic apical periodontitis. Patients were randomized into two equal groups. In the intervention group the functional and nonfunctional cusps were reduced until absence of contact was confirmed, while in the control group the occlusal surfaces were left intact. Standard endodontic treatment was performed in two visits using rotary nickel titanium files for shaping, 2.5% sodium hypochlorite for cleaning and lateral condensation technique with resin sealer for obturation. Pain was assessed preoperatively, then after 6, 12, 24 and 48 hours following instrumentation, then after 6 and 12 hours following obturation. Visual Analogue Scale (VAS) was used as the primary outcome measure. Patients were given a placebo to be administrated in case of severe pain and ibuprofen 400mg was prescribed in case of persistent pain.