To elucidate the mechanism of increased intraocular pressure in primary open-angle glaucoma (POAG), the protein profiles of aqueous humor obtained from POAG patients were compared with those of cataract patients as a control group. Aqueous humor proteins were analyzed by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and detected by the ultrasensitive silver staining technique. In 79% of the samples taken from POAG patients, protein bands of 140,000 or 160,000 daltons were stained, but none were stained from cataract patients. The presence of these protein bands revealed statistically significant differences between the two groups. Protein bands of 140,000 or 160,000 daltons were evenly visible at all ages in POAG patients, and the positivity of bands had no correlation with sex or initial intraocular pressure level. It is possible that the ultrastructural changes of the aqueous outflow pathway in POAG may be related to the changes in the aqueous protein, presence of 140,000 or 160,000 daltons protein bands.