Electrophysiologic studies were carried out on 11 patients with Holmes-Adie syndrome, 8 of whom had reduced or absent ankle jerks. Conduction velocities and evoked nerve and muscle compound action potentials in the peroneal, posterior tibial, and sural nerves were normal. The H reflex was absent (or virtually absent) in the patients with depressed reflexes. The amplitude of the composite Ia EPSP in single soleus motoneurons was estimated from changes in firing probability of voluntarily activated soleus motor units in response to stimulation of low threshold afferents in the tibial nerve. These amplitudes were used to test the afferent side of the reflex pathway. Composite group Ia EPSPs in Holmes-Aide patients with hyporeflexia were smaller than normal or absent, indicating that the areflexia in the Holmes-Aide syndrome is due to loss of large spindle afferents or reduced effectiveness of their monosynaptic connections to motoneurons.