Introduction : Although 5% hyperbaric lidocaine is to blame for TNS after its spinal anesthesia, it has been widely used for several years in Thailand. Recently, four cases of TNS after spinal anesthesia were reported at Chiang Mai University Hospital but its association with the use of 5% hyperbaric lidocaine was questionable. Objective : To compare the incidences of transient neurologic symptoms after spinal anesthesia with 5% hyperbaric lidocaine versus 0.5% hyperbaric bupivacaine. Study design : Double blinded randomized controlled trial. Method : Two hundred surgical patients scheduled for surgery under spinal anesthesia were randomly allocated to receive either 5% hyperbaric lidocaine or 0.5% hyperbaric bupivacine. On the following postoperative days, they were evaluated by the anesthesilogist who was unaware of the administered solutions for abnormal unaware of the administered solutions for abnormal neurological symptoms. Result : Seventeen patients in the 5% hyperbaric lidocaine group (17.17%) developed TNS while two patients in the 0.5% hyperbaric bupivacaine group (1.98%) developed TNS-like symptoms (p=0.003. Fisher's Exact test). The crude risk (95% CI) of TNS in the 5% lidocaine group is 10.262 (2.28, 46.1) and the adjusted risk (controlling for age, operation time and lithotomy position by the multiple logistic regression analysis) of TNS in this group is 8.82 (1.89, 41.04).Conclusion : The result has indicated that risk of TNS after spinal anesthesia is increased with the use of 5% hyperbaric lidocaine. Despite its risk of TNS, lidocaine is still useful in spinal anesthesia particularly for a short operative procedure. Therefore, the modified concentrations of lidocaine for spinal anesthesia is still required to reduce the risk of TRI and provide adequate analgesia during the operative procedure.