Eleven patients with acute diphtheritic neuritis were studied clinically and electrophysiologically. Decreased conduction velocity and increased distal motor latency were present in most patients within two weeks of the onset of neurological symptoms. Velocities later fell to approximately 45% of mean normal values. Wasting with electrophysiological evidence of denervation was present in 3 patients. Nerve conduction studies had returned to normal within 3 months in 8 patients. There was a striking dissociation between the time course of the clinical and the neurophysiological abnormalities: early in the illness, peripheral nerve conduction was normal in some patients despite the presence of severe weakness, and later, the maximum electrophysiological abnormalities were sometimes found after clinical recovery had commenced. The nature and sequence of the clinical and electrophysiological changes are in accord with the known pattern and distribution of the pathological changes in the disease.