Background: Sympathetic blocks with local anesthetics are used to differentiate sympathetically- maintained pain (SMP) from sympathetically-independent pain (SIP). However, systemic lidocaine is also used in the management of neuropathic pain. Therefore, there may be possibility of a false positive response in relieving their pain by systemic absorption of lidocaine following a diagnostic sympathetic block in patients with SIP. In this study, we measured the plasma lidocaine concentrations after a stellate ganglion block (SGB) using three volumes of 1% lidocaine. Methods: This prospective, crossover study was performed in 3 patients who experience sudden hearing loss and in 4 volunteers. Each person received SGB three times using three different volumes (6 ml, 12 ml and 16 ml) of 1% lidocaine at one week intervals. SGB was performed using a 23 G butterfly needle via a paratracheal approach by two persons. Two ml of venous blood was obtained from a prepared contra-lateral sided venous route at 1, 3, 5, 7, 10, 20 and 60 min after SGB. Plasma lidocaine level was analyzed by immunoassay. Results: Mean plasma lidocaine concentrations correlated well with the volumes of 1% lidocaine used in SGB; larger volumes showed higher concentrations (P < 0.01). Mean peak plasma concentrations were $1.08{pm}0.18$ in 6 ml, $1.90{pm}0.47$ in the 12 ml and $2.74{pm}0.67{mu}g/ml$ in the 16 ml groups (P < 0.01). The mean time to reach peak plasma concentration was not significantly different between the three groups. Conclusions: The peak plasma lidocaine concentrations in SGB using large volume were found to be similar to that of IV lidocaine infusion in the management of neuropathic pain. These data suggest that diagnostic sympathetic block may result in many false positive responses for SMP. Part of its effect may be related to systemic local anesthetic absorption and not to a sympathetic block. Therefore, physicians may be required to use optimal volumes and minimal concentration of local anesthetic in diagnostic sympathetic block procedures and also make a careful assessment of the performance of a permanent sympathetic block.