Background As intraocular pressure (IOP) and IOP fluctuation are known risk factors for glaucoma, it is important to understand the effects of high myopia on these ocular parameters. The aim of this study was to investigate if primary open-angle glaucoma (POAG) patients with high myopia exhibit higher IOP and greater IOP fluctuations at resting conditions over 24 hours. Methods We designed a prospective control clinical study. Eighty-two eyes of 82 high-tension Chinese POAG patients only on prostaglandin analogue locally were divided into 3 groups according to various myopic grades (<-6.0 D, n=27 and between -0.76 and -5.99 D, n=33) or without myopia (-0.75 to 0.75 D, n=22). Single time IOP at 10 am, mean corrected 24-hour IOP, mean corrected night IOP, 24-hour IOP fluctuation and IOPs of 10 am, 2 pm, 6 pm, 10 pm, 2 am, 6 am and 8 am were measured. Results The IOP was higher in POAG patients with high myopia over those POAG alone in three ways: the elevated IOP value was 0.65 mmHg measured in single time IOP at 10 am, 0.84 mmHg in mean corrected 24-hour IOP, 0.97 mmHg in mean corrected night IOP. The 24-hour IOP fluctuation was lower in the two myopia groups than in non-myopia group. Further, using repeated measurement analysis of variance, there was no statistical significance among groups regarding the IOPs at the seven time points (P=0.77) and there was no interaction between groups and time points (P=0.71), but the difference of IOPs at the seven time points in same group was statistically significant (P=0.01). Conclusion High-tension POAG patients with high myopia, even on pharmacological glaucoma therapy, still have higher IOP, but 24-hour IOP fluctuation at resting conditions was lower in these patients. Chin Med J 2012;125(7):1282-1286