Objective: To test the hypothesis that periodic limb movements (PLMs) are related to spinal flexor reflexes (FRs), the authors compared the state-dependent changes in FR excitability in 10 patients with restless legs syndrome (RLS) and PLMs with those from matched controls. Background: PLM is a disorder of motor control during sleep, frequently occurring in RLS. Clinically, PLMs resemble spinal FRs. Methods: FRs were obtained by electrically stimulating the medial plantar nerve and recording from antagonist leg and thigh muscles bilaterally. Results: Compared with controls, patients had significantly increased spinal cord excitability, as indicated by lower threshold and greater spatial spread of the FR, which was more prominent during sleep. Multiple late responses were seen during sleep in all patients and in some controls at higher threshold. The most prominent of these responses had a very long duration and a latency range of 250 to 800 msec, and because of its close temporal relationship to the FR stimulus, the authors considered it was a late, high-threshold component of the FR (FR3). The authors also found a similarity between the pattern of muscle recruitment and spatial spread of late components of the FR and those of spontaneous PLMs. Conclusions: The results support the hypothesis that PLMs in RLS and FRs share common spinal mechanisms and suggest that PLMs may result from enhanced spinal cord excitability in RLS patients. Because dopaminergic mechanisms are involved in spinal FR control, the results are consistent with the current view that RLS is a disorder of dopaminergic function.