To provide reference values for the oculomotor nerve, for example, fascicular area and myelinated fiber (MF) number and size distribution, in pathological states, and to determine whether oculomotor nerves from diabetic patients without a history of oculomotor palsy have subclinical structural alterations, the morphometric features of 15 control subjects and eight diabetic patients were evaluated at a proximal and a distal level. On average the control nerves had a fascicular area of 2.7 mm2, 22,311 MF, and a bimodal diameter distribution with peaks at approximately 5 to 6 and 10 to 11 microns and a range of 2 to 20 microns. In proximal nerve, glial bundles were found in one-half of the subjects and patients, with a peak of MF sizes that differed from fibers in nonglial areas. The nerves from diabetic patients departed from the nerves from control subjects in the following two respects: changed size distribution, suggesting atrophy of fibers, and microfasciculation of edge fibers in parts of the fascicles in one-half of the patients. We conclude that glial bundles are probably a normal variant. Microfasciculation occurred much more frequently in nerves from diabetic patients and may represent subclinical injury of unknown cause. The alteration in size distribution may reflect the diabetic state and is not necessarily a precursor to diabetic ophthalmoplegia.