Objectives. Although the load impedance of a pulse generator has a significant effect on battery life, the electrical impedance of contact arrays in spinal cord stimulation (SCS) has not been extensively studied. We sought to characterize the typical impedance values measured from common quadripolar percutaneous SCS contact arrays. Methods. In 36 patients undergoing percutaneous trial stimulation for various chronic pain conditions, bipolar impedance between adjacent contacts of 64 leads with 9mm center-to-center spacing was measured in two different vertebral level regions, cervical (C3-C7) and lower-thoracic (T7-T12). Multiple linear regression was applied to analyze the contribution of six variables to the biological tissue portion of the impedance (excluding the resistance of the lead wires). Results. The median impedance in the cervical region (351 +/- 90 Ohm was significantly lower (36%, p<0.001) than in the lower-thoracic region (547 +/- 151 Ohm). In addition, time since implant had a weaker but still significant effect on tissue impedance. Conclusions. Results from finite-difference mathematical modeling of SCS suggest that the difference in tissue impedance related to vertebral level may be due to the dorsoventral position of the lead in the epidural space. The presence of a larger space between the triangularly shaped dorsal part of the vertebral arch and the round shape of the dural sac in the lower-thoracic region increases the likelyhood that the stimulating lead will not make dural contact, and thus "see" an increased impedance from the surrounding epidural fat. This implies that the energy requirements for stimulation in the thoracic region will be higher than in the cervical region, at least during the acute phase of implant.