We compared histologic features of sural nerve biopsies in 14 patients with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) with those in other forms of neuropathy. In CIDP endoneurial pericapillary cellular infiltrates were found in 4 patients (29%), onion bulbs in 5 patients (36%), and predominant demyelination in 7 patients (50%). None of these abnormalities was specific, but cellular infiltrates and onion bulbs appear to be diagnostically useful when combined with clinical information. To detect macrophage infiltration of myelin, cell nuclei were counter-stained in 20 teased fiber preparations. Nine patients with CIDP had a significantly higher mean number of cells per centimeter of teased fiber than 11 patients with other neuropathies. Despite overlap, significant infiltration of myelin detected by this method suggests CIDP in an appropriate clinical setting.