The number of myelinated fibers per square millimeter of fasciculus gracilis, and of large and small myelinated fibers per square millimeter of fasciculus gracilis, at the C3 and T5 segements of spinal cord from 22 persons of different ages have been estimated, with a 95% confidence limit, assisted by semi-automatic methods of measurement and using programmed calculation and plotting. In addition, the histograms for diameter of myelinated fibers and mean diameter were obtained. At C3, the numbers of myelinated fibers ranged from 19,647/mm2 to 35,773/mm2, with a mean of 25,267/mm2. The number is less with older age. At T5, numbers of myelinated fibers ranged from 17,804/mm2 to 29,486/mm2, with a mean of 23,069/mm2. The number of myelinated fibers was greater at C3 than at T5 in subjects under 45. The mean diameter of myelinated fibers at C3 was smaller than that at T5. The normative data presented here should be useful for comparing with similarly derived data from patients with diseases of peripheral nerves, in evaluating the three-dimensional morphologic alteration of primary afferent neurons.