When confronted with a patient with a brachial plexus injury, how often as neurosurgeons do we consult an atlas to confirm the anatomy of the brachial plexus and then attempt to establish the location of the lesion? Similar difficulties are encountered with lumbar and sacral plexus lesions. In a project organized to assist the neurosurgeon in this time-consuming task, a computer program that can rapidly determine the site of a lesion in a brachial, lumbar, or sacral plexus injury was crated. Using known anatomical pathways (37 clinically relevant upper and 20 lower extremity muscle innervations), and relying solely upon the neurological motor examination, rapid computer-assisted diagnosis is possible. When more than one final common pathway lesion occurs (for example, multiple root avulsions of the brachial plexus), possible lesion sites can be obtained. An interactive dialogue between the user and the program helps to determine the location of the lesion. The program can be run on any IBM-compatible personal computer and is presented as an instrument that provides assistance in cases of complex peripheral nerve injuries, when expert consultants are unavailable. In addition, it can be used as an aid to learning and as a review of basic neuroanatomy.