The objective of the study was to determine the diagnostic value of features of demyelination and inflammation in sural nerve biopsy specimens of patients with chronic inflammatory demyelinating polyneuropathy (CIDP). The features of (i) demyelination, (ii) axonal de- and regeneration and (iii) inflammation were investigated by measuring the number of onion bulbs, g ratio (axon diameter/total nerve fibre diameter), myelinated nerve fibre density, number of clusters and endoneurial area in 21 patients with CIDP, as well as in 13 patients with chronic idiopathic axonal polyneuropathy (CIAP) and six autopsy controls. In addition, teased fibres were classified and lengths of internodes measured. We found no difference in demyelinating features between patients with CIDP and CIAP, as the percentage of fibres with segmental de- and remyelination and the number of onion bulbs were similar in both polyneuropathy groups. The g ratio, expected to be higher in a demyelinating disease due to thinner myelin sheaths, was significantly lower in CIDP than CIAP. Evidence for axonal degeneration was found in both CIDP and CIAP, as both showed a decrease in myelinated nerve fibre density. There was no evidence of endoneurial oedema in CIDP, as the endoneurial area did not differ between CIDP, CIAP and the autopsy controls. Although significant differences of features of demyelination, axonal degeneration and inflammation were found in sural nerve biopsy specimens, there was a considerable overlap between abnormalities in CIDP and CIAP patients. In the majority of patients, quantitative analysis of light microscopical abnormalities in sural nerves was similar in CIDP and CIAP. Therefore, a sural nerve biopsy is of limited diagnostic value in CIDP.