The aim of the present study was to assess the effect of occlusal reduction on post-operative pain after endodontic instrumentation and obturation in patients with symptomatic irreversible pulpitis and symptomatic apical periodontitis in mandibular posterior teeth.Subjects and Methods This study is a prospective, parallel, randomized, double blinded clinical trial. The subjects of this study were allocated randomly into two groups; a control group and an intervention group. Patients included in this study were chosen according to the following inclusion and exclusion criteria: (table1). They presented with signs and symptoms of symptomatic irreversible pulpitis and symptomatic apical periodontitis. Their chief complaint was pain on biting and one or more of the following clinical signs and symptoms were described: Severe to moderate throbbing pain, hypersensitivity to cold and hot beverages, spontaneous pain or pain that increase at bed time. All patients were selected and treated in the Endodontic clinic of Faculty of Oral and Dental medicine, Cairo University. The operator was postgraduate student in the Department of Endodontics.ResultsData for mean age, gender and tooth type for different tested groups.There was no statistically significant difference in mean age values (p=0.157), gender distribution (p=0.546), and in the tooth type distribution (p=1.00) between the two groups.