The area of the unrolled myelin sheet of internodes of myelinated fibers (MF) of peripheral nerve is thought to be determined by axonal caliber and internodal length. We studied the effect of a focal increase of axonal caliber due to the deposition of polyglucosan bodies (PGB), amylopectin-like glucose polymers, on number of myelin lamellae (NL), interlamellar distance (periodicity), and myelin spiral length (MSL) from a sural nerve biopsy specimen of a patient with chronic inflammatory demyelinating polyneuropathy. Axonal area, NL, periodicity, and MSL were estimated within internodes of MF above, at, and below PGB. The axon caliber at the level of the PGB was significantly (P less than 0.002) increased when the PGB was included. At the PGB, NL and their periodicity were not significantly different from those above or below the PGB. The MSL was significantly longer overlying the PGB than it was in the same internode above or below the PGB. Because slippage or stretching of the myelin sheath as well as movement of molecular constituents of myelin is not likely over large distances, localized biosynthesis and assembly of new myelin may explain this increase of MSL.