F-wave persistence of the deep peroneal nerve was studied in 41 patients with L5-S1 compressive root lesion (22 with radicular pain (RP) and 19 with motor and/or sensory (MS) signs), and in 26 normal control subjects. We used specific conditions, i.e., surface electrodes, stronger stimulus and facilitating maneuver in order to enhance the frequency of F-response appearance. EMG and nerve conduction studies were done F-latency was also measured on both sides of the patients. In the affected sides, both RP and MS, F-persistence's mean value was 56.1 and 31.0 respectively, which was significantly (p < 0.01) lower than controls (mean 83.2), and showed a significant correlation with the extensor digitorum brevis maximal interference pattern. These findings possibly reflect demyelination and/or axonal damage of lower motoneuron. Reduced F-persistence was found in 22 selected patient sides with unequivocal dorsal root damage only (mean 56.7, p < 0.01). This finding could be the result of a reduced lower motoneuron excitability because of the disruption of the circuit retaining the muscle tone. The diagnostic utility of F-persistence, F-latency and EMG was also tested. EMG and F-persistence were found statistically equal while F-persistence was found with a significant preponderance in comparison with F-latency. Finally, EMG was found more sensitive than F-latency in the patient sides with neurologic deficits.