Background: Regarding the severity of post cesarean section pain and its known complications if remained untreated, and also the availability of petidine, this study has been carried out on patients referring for cesarean section to 1mam Hossein and Shohada-e-Tajsish Hospital during the academic year 1999, to determine the efficacy of intrathecal petidine added to 5% lidocaine on post cesarean section pain. Materials and methods: This clinical trial of sequential double-blinded type has performed on 80 patients distributed randomly in two groups of case (40) and control (40). The surgical procedure, spinal anesthesia technique and patients' characteristics were similar in both groups. All the patients have received 500 ml of ringer lactate intravenously and then 75 mg of 5% lidocaine (1.5 ml) was injected through L4-Ls in lateral decubitus position with special needles (No. 25) prepared for this purpose. At the same time, the case group has received 0.5 mg/kg of 5% petidine (0.6-0.8 ml), however, the control group was given the same amount of normal saline. Patients were observed carefully during and after the operation, then within the first day, they have been examined for vital sign and post-operative pain, using V AS and OAS criteria. Results: Patients were weighed 61.3±1.1 kg. Mean age of case and control group was 26.3±4.4 and 27.2±5.2 years old, respectively. The number of pregnancy was 2.8±1.4 and 2.6±1.6 in the case and control group, respectively, however, the number of delivery for these groups was 2.1±1.2 and 2.1±1.1, respectively, According to the VAS criteria, in the case group, 10% were free of pain, 50% have reported mild pain, 36.7% felt moderate pain and the remaining 3.3% suffering from severe pain whereas in the control group moderate pain was reported in 10%, severe pain in 76.7% and extremely severe pain in 13.3% of the patients (p<0.0005). In the meantime, regarding GAS criteria, in the case group, 13.3% were free of pain, 56.7% have reported mild and the remaining 30% had moderate pain, whereas in the other group, moderate pain was reported 13.3%, severe pain 66.7% and extremely severe pain 20% (p<0.0005). No significant statistically difference has been shown regarding the first and fifth apgar score between our groups (p< 0.2). Side effects were: itching (16.7% vs. 0), nausea (70% vs. 43.3%), vomiting (43.3% vs. 30%), hypotension during the operation (66.7% vs. 50%), bradycardia (20% vs. 13.3%). (First figures referred to the case group and second to the control). Conclusion: Adding intrathecal petidine could significantly eliminate post-operative pain, and its side effects including itching, hypotension, nausea and vomiting are treatable. These criteria have been reviewed by anesthesiologists and checked by means of X2 and Fischer tests.