OBJECTIVE To characterize the dysautonomia associated with acute sensory motor neuropathy and to discuss the classification of acute autonomic neuropathy. METHODS Case series. METHODS Sympathetic skin response. Local sweat response to acetylcholine. Norepinephrine infusion test and acetylcholinesterase histochemistry of sural nerve biopsy specimens in addition to making conventional analyses of myelinated and unmyelinated fibers. RESULTS In 12 patients with chronic neuropathy, acetylcholinesterase-positive fiber density and plantar sympathetic skin response size were well correlated, but in the two patients with acute autonomic sensory and motor neuropathy, there were discrepancies, acetylcholinesterase-positive fiber density being well preserved and sympathetic skin responses being absent. Histologic and electrophysiologic results indicated primary demyelination of the myelinated fibers. In contrast, previous studies of acute autonomic sensory and motor neuropathy reported dysfunction of the sympathetic postganglionic fibers and axonopathic change in myelinated fibers, poor recovery from dysautonomia. CONCLUSIONS Dysautonomia with acute idiopathic neuropathy can be divided into two categories--postganglionic axonopathic and preganglionic demyelinating types of the sympathetic efferent pathways. The recovery from dysautonomia produced by the former lesion is poor, but recovery is better for that produced by the latter lesion.