OBJECTIVE The aim of this study was to compare diurnal and nocturnal intraocular pressure (IOP) fluctuations before and after trabeculectomy, and to evaluate the potential of trabeculectomy to even out IOP peaks. METHODS Retrospectively, 35 diurnal and nocturnal IOP curves of patients (35 eyes) who underwent trabeculectomy were analyzed. Diurnal and nocturnal IOP profiles were recorded from 06:00 to 23:59 and 00:00 to 06:00, respectively. Follow-up was at least 300 days. Two criteria for success were defined: (1) IOP ≤21 mm Hg and at least a 20% IOP reduction from baseline; (2) <18 mm Hg without medication. RESULTS The preoperative maximum diurnal and nocturnal IOP (IOP(max)) were 26.5 ± 5.9 and 23.4 ± 5.2 mm Hg, respectively. The postoperative diurnal and nocturnal IOP(max) were 16 ± 4.4 and 16 ± 5.4 mm Hg, respectively. Pre- to postoperative IOP reductions were statistically different (day 40% and night 32%; p < 0.001). Diurnal fluctuation was reduced significantly from 12.1 ± 4.2 mm Hg preoperatively to 5.6 ± 2.2 mm Hg postoperatively (reduction of 54%), and nocturnal fluctuation from 7.1 ± 4.5 to 3.9 ± 4.1 mm Hg (reduction of 46%, statistically insignificant due to large SD), respectively. Success criteria 1 and 2 were achieved in 71 and 54% of patients at daytime and in 63 and 57% at nighttime (no statistically significant difference detectable). Follow-up was 2.1 ± 1.7 years. CONCLUSIONS Trabeculectomy achieves a leveling of IOP(max) in the diurnal-nocturnal comparison. The preoperative fluctuations could be nearly bisected. Relating to the success criteria, the statistical analysis did not reveal a difference in the diurnal-nocturnal comparison.