In myelinated fibers of the sural nerves in Charcot-Marie-Tooth disease 1A (CMT 1A), with peripheral myelin protein (PMP) 22 gene duplication, it has been controversial whether axonal attenuation occurs or the myelin sheath thickens. Therefore, the relationship between the transverse axonal area and the number of myelin lamellae was morphometrically studied in myelinated fibers of the sural nerves in CMT 1A with PMP22 gene duplication to re-evaluate such relationships as are revealed in hereditary motor sensory neuropathy, type 1 without genetic diagnosis. In electron microscopic studies both the axonal circumference and natural log (ln) axonal area were significantly (P < 0.01) smaller in the CMT 1A group (n = 8) than in the control group (n = 9). Myelin lamellae numbers, however, were similar in both groups. In a regression analysis relating ln axonal areas to the numbers of myelin lamellae, ln axonal area in the CMT 1A group was significantly (P < 0.05) smaller than in the control group in myelinated fibers with the numbers of myelin lamellae equal to or over 40. Therefore, it was concluded that the axonal area was smaller in large-diameter myelinated fibers in the CMT 1A group than in the control group. We speculated on the existence of axonal attenuation due to the impaired effect of Schwann cells on myelinated axons in CMT 1A with PMP22 gene duplication.