Aims: To compare post-operative pain following 25-gauge (25G) and 20-gauge (20G) vitrectomy in the first week following surgery.Methods: The study was a pilot randomised controlled trial with patients masked to the treatment allocation. Postoperative pain was assessed using both a visual scale and verbal pain scores for 1 week following surgery. Additional data collected included intraocular pressure (IOP), time taken to perform the surgical procedure, per-operative and post-operative complications, and dropout rates.Results: Forty patients were recruited for the study: 21 randomised to 20G vitrectomy and 19 to 25G. In the first 12 h following surgery, presence of significant postoperative pain (defined as >1 cm on a visual analogue scale) was similar in both 20G (50%) and 25G (53%) patients. In the first week following surgery, 38 of the 527 scores (7.2%) were >1 (median 2.1, IQR 1.3-3) cm; however, there was evidence that "significant pain'' was experienced more commonly in the 20G group. There was no statistical difference in the time taken to complete the surgical procedure, although in the 25G group the time from first incision to the start of vitrectomy was significantly shorter (p = 0.043) and in the 20G group the time taken to complete the vitrectomy was less (p = 0.047). Post-operative hypotony (IOP