Progression periods for Guillain-Barré syndrome (GBS) differ from those of chronic inflammatory demyelinating polyneuropathy (CIDP), but physicians could classify patients with CIDP within 4 weeks of onset as GBS. We studied and report the frequency of GBS patients who were later diagnosed as CIDP (11/663, 2%). Plasmapheresis or intravenous immunoglobulin transiently improved all the 11 patients, who 11 progressed slowly or had a relapse beyond the 8 weeks, and the other 2 suffered a relapse between 4 and 8 weeks from the onset. Three patients had had an antecedent infectious illness. CSF albumino-cytological dissociation was detected in 6 patients within 2 weeks of onset. Recognition of the existence of such patients is important for the early diagnosis and treatment of those patients with CIDP for whom GBS has been diagnosed at onset.