Nitric oxide (NO) has effects on the regulation of aqueous humor dynamics, but the exact mechanism is not yet established. To investigate the possible roles of NO in glaucoma, we determined NO levels in aqueous humor and plasma in glaucoma patients and a control group, cataract patients. The study is an open trial with purposed sampling. One hundred fifty-two patients, including 87 glaucoma patients and 65 cataract patients from two medical centers, were recruited. NO levels in the samples were measured by a chemiluminescence assay. We found that, although the mean aqueous humor NO level (mean +/- SEM) was higher in the glaucoma patients than in the cataract patients (39.7 +/- 1.5 microM vs. 35.5 +/- 1.3 microM, p < 0.05), NO levels varied significantly in different types of glaucoma. The juvenile glaucoma patients had the lowest mean NO level (8.4 +/- 0.9 microM), while the acute angle-closure glaucoma and neovascular glaucoma patients had the highest mean NO levels (64.8 +/- 7.6 microM, 67.3 +/- 8.2 microM). In comparison, the mean plasma NO level in the glaucoma patients was not statistically different from that in the cataract patients (14.1 +/- 1.2 microM vs. 13.9 +/- 1.1 microM, p = 0.91). Our data may provide information for applying NO-mimicking nitrovasodilators in the treatment of glaucoma.