Our knowledge of the specific root innervation of skeletal muscles is derived from accumulated clinical experience. While performing selective posterior rhizotomy for treatment of spasticity in children with cerebral palsy, we made direct electrophysiologic measurement of the root innervation of the lower extremity. We stimulated ventral roots from L2 to S2 while recording from all muscles simultaneously. The size of the evoked compound muscle action potential was used as an indication of the amount of innervation derived from stimulation of a given spinal root. We found the major root innervation for the 8 muscles studied to be: adductor longus, vastus medialis, and vastus lateralis, L3; tibialis anterior; L4; peroneus longus, L5; and medial gastrocnemius, lateral gastrocnemius, and gluteus maximus, S1. In general, each muscle received innervation from 3 or more roots. Prefixed or postfixed innervation patterns were found in 27.9% of legs examined, and there was asymmetry of innervation in 29.8%. We conclude that the segmental innervation of lower extremity muscles is broader than previously thought. Anomalous innervation occurs so frequently that caution should be used in attributing any pattern of clinical or EMG findings to a specific spinal level.