The herpes simplex viruses (HSVs) are ubiquitous pathogens, capable of causing either asymptomatic or symptomatic disease. Herpes simplex virus keratitis (HSK) is the most frequent infectious cause of blindness in developed countries.1 Liesegang et al.2 have reported an incidence of 8.4 primary ocular HSV infections per 100,000 person-years. Moreover, further investigation of the natural history of HSK shows a recurrence rate, after the first episode, of 9.6% at 1 year, 22.9% at 2 years, and 63.2% at 20 years.2 The cornea is the most densely innervated tissue of the body, supplied by the terminal branches of the ophthalmic division of the trigeminal nerve as ciliary nerves.3,4 Corneal nerves penetrate the corneal periphery in a radial distribution and form the subbasal nerve plexus between the Bowman's layer and the basal epithelium.3 Corneal innervation provides protective and trophic functions and regulates epithelial integrity, proliferation, and wound healing.5,6 Recently, in vivo confocal microscopy (IVCM) has greatly advanced the microscopic evaluation of ocular structures. Specifically, the corneal subbasal nerve plexus and endothelial layers have been shown to be important indicators of corneal health and disease.7–10 One of the sequelae of HSK is neurotrophic keratopathy, caused by impaired corneal innervation.1,11 The insufficient supply of trophic neural factors and impairment of the tear and blink reflexes are caused by loss of corneal innervation.2,12 More recent studies5,6,13,14 have shown changes in subbasal corneal nerves in patients with early-stage Fuchs' endothelial corneal cell dystrophy (FECD). These findings suggest that corneal nerve alterations may be involved not only in the pathogenesis of FECD, but also potentially in other diseases resulting in decreased endothelial cell density. The embryologic similarities between endothelium and neuronal cells may support the theory that FECD has a neurodegenerative component.7–10,15 Further, Koh et al.16,17 have reported that the neuropeptide vasoactive intestinal peptide (VIP) has a protective effect on the corneal endothelium, playing an active role in maintaining the differentiated state and suppressing apoptosis in endothelial cells. Finally, in a recent preclinical study, we have demonstrated that trigeminal nerve axotomy results in a continuous loss of corneal endothelial cells centrally and peripherally, which correlates with a significant decrease in VIP after axotomy. Loss of endothelial cells is prevented through daily replenishment of the neuropeptide VIP (Yamaguchi T, et al. IOVS 2014;55:ARVO E-Abstract 2077). Thus, in the current study, hypothesizing that corneal nerves may mediate endothelial cell homeostasis, we performed IVCM in patients with a unilateral HSK, in order to analyze bilateral corneal endothelial cell density (ECD), as well as its correlation to subbasal nerve morphology and function.