Using MRI, we investigated the morphology and blood-nerve barrier function of the peripheral nerve trunk in 10 patients with chronic inflammatory demyelinating polyneuropathy (CIDP). Eight patients had a focal demyelinative segment in the median or ulnar nerve trunk that was defined by conduction block or abnormal temporal dispersion over a short distance. These demyelinative foci showed nerve enlargement with high signal intensity on proton or T2-weighted images. In four patients with progressive illness or relapse, the enlarged segment showed gadolinium enhancement that disappeared during remission induced by immune therapies. The other four were in the steady phase and showed no gadolinium enhancement of the enlarged nerves. The two patients who showed conduction slowing, but no focal demyelinative focus, had neither nerve enlargement nor gadolinium enhancement. In CIDP, focal conduction abnormalities correlate well with anatomic changes that suggest intermittent, repeated inflammation associated with the breakdown of the blood-nerve barrier.