Purpose: To evaluate the daytime fluctuation of intraocular pressure (IOP) in patients with primary angle-closure glaucoma (PACG) after trabeculectomy.Participants and Methods: A total of 176 patients with PACG participated in a clinical trial of trabeculectomy with or without releasable sutures. Applanation IOP was measured at 5, 7, and 10 am, and 2, 6, and 10 pm at 3 months posttrabeculectomy. We documented the mean, peak, and trough IOPs, determined the fluctuation of daytime IOP, and explored the associations of IOP fluctuation with baseline factors.Results: IOP measurements were obtained in 173 patients. The mean daytime IOP was 13.2 +/- 3.7 mm Hg; mean peak IOP 15.1 +/- 4.1 mm Hg, mean trough IOP 11.3 +/- 3.5 mm Hg, and mean fluctuation 3.8 +/- 2.1 mm Hg. Fluctuation was positively correlated with peak (r=0.528, R-2=0.28, P<0.001) and mean IOP (r=0.278, R-2=0.08, P<0.001), but not with the trough IOP (r=0.015, P=0.843). Fluctuation was lower with extent of bleb (0.6 mm Hg/unit increase in extent; 95% CI, 0.1-1.2 mm Hg) and in blebs with microcysts (1.1 mm Hg less fluctuation; 95% CI, 0.2-1.9 mm Hg). Fluctuation was not associated with sex, age, baseline IOP, extent of peripheral anterior synechia or number of glaucoma medications before surgery, mean deviation of the visual field, vertical cup:disc ratio, or the use of releasable sutures.Conclusions: The mean fluctuation of daytime IOP after trabeculectomy for PACG was about 4 mm Hg. The fluctuation was positively associated with higher peak and mean IOP and negatively associated with extent of bleb and presence of microcysts.