The diagnosis of acute Guillain-Barré syndrome and chronic inflammatory demyelinating polyradiculoneuropathy is based on clinical characteristics, abnormalities on nerve conduction studies, and nerve biopsy specimens indicating demyelination. Inflammation and edema are also common findings in nerve specimens. Immunotherapy is helpful in these dysimmune conditions. Occasionally the diagnosis is difficult to make, particularly when electrophysiological testing or nerve biopsy findings are not characteristic. The authors found contrast enhancement of lumbosacral roots in patients with chronic inflammatory demyelinating polyradiculoneuropathy and Guillain-Barré syndrome, but not in those with other demyelinating neuropathies. Contrast-enhanced magnetic resonance imaging could be a useful tool in the diagnosis of the dysimmune inflammatory neuropathies.