We studied the cerebrospinal fluid (CSF) in patients with acute (AIDP) and chronic inflammatory demyelinating poliradiculoneuropathy (CIDP). In order to assess its possible contribution in establishing the diagnostic approach, we analyzed the cell content, concentrations of total proteins, albumin and IgG, CSF/serum albumin quotient and the qualitative study of IgG. Sixteen patients with AIDP and 16 patients with CIDP, fulfilling the accepted diagnostic criteria, were included in this study. CSF features were not used as exclusion criteria. The cell count and concentration of total proteins were determined by standard procedures. CSF albumin was performed and IgG were done by single radial immunodiffusion, and the qualitative analysis of IgG by agarose isoelectric focusing of unconcentrated CSF. The incidence of elevated levels of total CSF proteins was identical in patients with AIDP and CIDP, reaching 94%. The mean level of CSF protein was similar in patients with AIDP and CIDP, 1100 and 1150 mg/l, respectively. The cell count was elevated in 3 (19%) patients with AIDP and normal in patients with CIDP. There were no significant differences in the mean level of any of CSF parameters between patients with AIDP and CIDP. Local synthesis of CSF oligoclonal IgG was found in 2 patients with CIDP and was not detected in patients with AIDP. No association between CSF and clinical parameters was recorded. The results of this study confirm that an elevated CSF protein and low CSF cell count are features highly supportive of the diagnosis of both AIDP and CIDP. The most intriguing finding is the detection of locally produced CSF oligoclonal IgG which may suggest the associated inflammation within the central nervous system in a number of patients with CIDP, and furthermore a pathogenic link between CIDP and multiple sclerosis.