This prospective randomized controlled trial aimed to compare changes in intraocular pressure in three different anti-inflammatory regimens following trabeculectomy. Sixty-nine patients were randomized to receive either postoperative prophylaxis with topical preservative-free dexamethasone (DEX), diclofenac (DICLO), or their combination (DEX+DICLO). Our main outcome measure was an intraocular pressure (IOP) change of a minimum 4 mmHg following the withdrawal of anti-inflammatory prophylaxis 9 weeks after trabeculectomy. We found that the IOP decreased ≥ 4 mmHg in 18.6% of eyes after cessation of the topical steroid DEX (n = 3/22) and DEX+DICLO (n = 5/21), whereas a decrease in IOP was not observed in the DICLO group. In conclusion, IOP decreased in nearly 1/5 of patients after cessation of topical steroidal anti-inflammatory prophylaxis after trabeculectomy. This points toward a steroid-induced increase in IOP even after trabeculectomy. Thus, increased postoperative IOP may be related to steroid use, and the success or failure of a trabeculectomy cannot be fully evaluated before anti-inflammatory prophylaxis with steroids is stopped or changed to non-steroidal eye drops.