Purpose.MUC5AC has been identified as a major secretory mucin of conjunctival goblet cells and precorneal tear film. However, no method has been reported to quantify MUC5AC protein in human tears. The objective of this study was to establish a method to measure the amount of MUC5AC in human tears and to correlate the amount of MUC5AC with age, gender, and dry eye diseases.Methods.A goat antibody was raised to synthetic peptides corresponding to nonglycosylated epitopes of human MUC5AC mucin. This antibody and a horseradish peroxidase-coupled second antibody were used to develop a quantitative immunoassay to measure the MUC5AC concentration of tear samples collected on Schirmer strips. Porcine stomach mucin was used as a standard for the assay. The chemiluminescent MUC5AC signal was digitized and quantified. Tear samples from 19 healthy volunteers and 31 clinically diagnosed dry eye patients were analyzed.Results.MUC5AC concentration in human tears ranged from undetectable to more than 200 &mgr;g/mL porcine stomach mucin equivalent. In the healthy population, low, moderate, and high concentrations were found in the tear samples from younger and older persons and from both men and women. The mean MUC5AC content in tears was lower in the dry eye patients than in the age- and gender-matched healthy individuals.Conclusions.A method was established to quantify MUC5AC in human tear samples obtained on Schirmer strips. There was no correlation between the amount of MUC5AC and age or gender in the healthy population. Dry eye disease patients, however, typically showed reduced concentrations of soluble MUC5AC in the tear film.