The role of nitric oxide (NO) in ocular surface diseases remains unknown. We investigated the conditions leading to increase NO generation in tears and the main sources of ocular surface tissue. We evaluated the possibility of a dual action (cell survival or cell death) depending on the amount of NO. The concentration of nitrite plus nitrate, the stable end-product of NO, was measured in the tears of various ocular surface diseases. We also examined the main source of nitric oxide synthase (NOS) using immunohistochemical staining & Western blot analysis. When cultured human corneal fibroblasts were treated with NO producing donor with or without serum, the viability of cells was studied. We found that sources of NO in ocular surface tissue primarily included corneal epithelium, fibroblasts, endothelium and inflammatory cells. Three forms of NOS (eNOS, bNOS, & iNOS) were expressed in experimentally induced inflammation. Cell death by NO revealed TUNEL positive staining, however in the EM finding, this NO specific cell death was an atypical necrosis showing perinuclear large vacuolization and mitochondrial swelling. In the fibroblasts culture system, the NO donor (SNAP, S-nitroso-N-acetyl-D, L-penicillamine) prevented the death of corneal fibroblasts caused by serum deprivation in a dose dependent manner up to 500(m SNAP, although a higher dose decreased cell viability. This study suggested that NO might act as a double-edged sword in ocular surface disease depending on the degree of inflammatory condition related with NO concentration.