Sciatic nerve lesions are usually painful and secondary to compression, trauma, infarction or part of a systemic illness. The etiology is usually defined by radiographic or blood studies, or by exploratory surgery. In rare cases, as the one being presented, there is clear clinical and electrophysiological evidence for a lesion of the sciatic nerve, but no morphological correlate or defined etiology. These idiopathic sciatic mononeuropathies seem to occur in the nerves of the legs in young adults.