Background: Keratoconjunctivitis sicca (KCS) is a chronic ocular disease of both dogs and humans that can result in ocular discomfort, corneal opacification, and vision loss. Lacritin, a protein found in the tears of many species, has been shown to play a role in lacrimation and corneal health. Because of its role as a potential lacrimostimulant, assessment of endogenous lacritin levels could reveal a correlation between lacritin and tear production in the dog. Objectives: To determine if tear lacritin concentrations are decreased in canine eyes affected by KCS. Animals: 58 client-owned dogs (tear samples from 55 eyes with normal tear production and 55 eyes diagnosed with KCS). Methods: All eyes underwent an ophthalmic exam, including Schirmer Tear Testing (STT), anterior segment assessment, and tear sample collection. Tear samples were evaluated for their total protein concentrations via BCA assay and lacritin concentrations via ELISA. Results: Total protein of canine tears is increased in KCS-affected eyes as compared to normal eyes. Tear lacritin as a component of total tear protein is significantly decreased in tears from KCS-affected eyes. When measured as a concentration (mass per volume of aqueous tears), lacritin is not significantly different between KCS-affected eyes and normal eyes, nor were they strongly correlated to STT values. Conclusions and Clinical Importance: Total tear protein levels were significantly increased in canine KCS. When quantified as a proportion of total tear sample protein, tear lacritin levels are decreased in KCS-affected eyes. Relative to tear volume, tear lacritin levels are not significantly different between KCS-affected eyes and normal eyes. Assessment of lacritin supplementation in canine KCS is warranted to evaluate potential effects on lacrimation and ocular surface health. / Master of Science / Keratoconjunctivitis sicca describes a syndrome of inadequate pre-ocular tear film, due to decreased quantity or quality of tears on the ocular surface. A multitude of clinical signs and sequelae result, including ocular discomfort, ocular discharge, and corneal opacification that may lead to vision loss. Current therapies aim at improving endogenous tear production; standard therapy, however, may not significantly improve disease in up to 30% of canine patients and 40% of human patients. Development of additional therapies could improve the vision and comfort of patients with KCS. Lacritin, a protein found in the tears of many species, has been shown to play a role in lacrimation and corneal health. Because of its role as a potential stimulant of tear production, assessment of endogenous lacritin levels could reveal a correlation between lacritin and tear production in the dog. This project aims to compare tear lacritin concentrations in dogs with normal tear production versus dogs with KCS. Tear samples were collected from normal dogs (n=55) and those affected with KCS (n=55). Samples were analyzed for their total tear protein levels via BCA assay and for their lacritin levels via ELISA analysis. Total tear protein levels and tear lacritin levels were compared to Schirmer Tear Test results, a routine method of diagnosing KCS in dogs. Tear lacritin was significantly decreased in KCS, relative to the quantity of total tear proteins. As a portion of the total volume of tears, lacritin was not significantly different in KCS-affected eyes as compared to normal eyes. Further investigation is warranted to determine the effects lacritin supplementation may have on canine KCS, in regard to tear production and clinical signs of KCS.