BACKGROUND: Radiofrequency (RF) lesioning is one of the most frequently used neurolytic techniques for the relief of pain. Technical advances enables one to make a more reliable and reproducible lesion production. If the size at different parameters is known, the maximal effects and reduction of the side effects could be achieved. However, its size can not be measured systematically using thermocouple (TC) electrodes. 'Differential selection of pain fibers' was proposed but there was not any neuropathologic evidence. We studied pathologic changes produced with various sizes and shapes of electrodes using different parameters. METHODS: Using fresh egg white, RF lesions were produced by four different electrodes at 65, 70, 75, 80 and 90degreesC. At each temperature, Photographs were taken at 10, 20, 30, 40, 50, 60, 90 and 120 seconds. Using the sciatic nerve of the rats, we performed RF lesioning utilizing two different electrodes at 70, 80 and 90degreesC and dissected them 1, 7, and 30 days after treatment. The pathologic changes of lesions were studied and analyzed by applying a quantitative experimental scoring system on the light and electron microscopy (LM and EM). RESULTS: The lesion size increased with a higher temperature and larger electrode. Among the electrodes with the same thickness, the lesion size with the longer electrode was larger than the shorter one. In a histopathologic study, there were significant changes with time, but no significant changes with different electrode and temperature. On electron microscope (EM), large myelinated fibers were relatively intact on RF lesion of 80degreesC after 1 day. 7 days after treatment, there were significant inflammatory cell infiltration and axonal regeneration. At 30 days after the same treatment, there were relatively large amount of small myelinated fibers and unmyelinated fibers. CONCLUSIONS: We measured the lesions systematically with different parameters expecting the result can be used as the reference for the RF lesion. There were no histopathologic differences on LM at different electrodes and temperatures. But we found the evidence of 'differential selection of pain fibers' with 22 gauge electrode at 80degreesC. And also we found the axonal regeneration as early as 1 week later. We learned the neuropathic pain can be induced by pathologic changes, such as bleb formation, inflammatory cell infiltration and predominance of small myelinated and unmyelinated fibers.