Recurrent inhibitory pathways are powerful modulators of motor neuron excitability. Renshaw cell activation can both inhibit homologous motor neurons and disinhibit antagonists. In spastic spinal-cord-injured patients, recurrent inhibition is consistently increased, and clinical reductions in spasticity are associated with reduced recurrent inhibition. In this study, we evaluated 12 spastic patients with amyotrophic lateral sclerosis (ALS) to see whether a similar mechanism was operating. In contrast to spinal-cord-injured patients, spastic patients with ALS showed strikingly reduced recurrent inhibition, as assessed by a conditioned H-reflex technique, which produces a response (H') whose amplitude is inversely correlated with activity in recurrent inhibitory pathways. The mean ratio of the maximum H' response to the maximum H-reflex response (H'/H ratio) was 0.55, significantly greater than the ratio seen in normal subjects. Amplitude of the H' correlated with amplitude of the Achilles tendon reflex. Thus, in patients with classical ALS, recurrent inhibition appears to be abnormally reduced compared with control subjects, suggesting a different physiology for spasticity in this setting than in spinal cord transection.