Background: To assess the efficacy and to determine the risk factors of trabeculectomy with mitomycin C (MMC) in eyes with neovascular glaucoma (NVG) secondary to diabetic retinopathy. Methods: Kaplan-Meier survival analysis of the surgical outcome was performed on 35 eyes with NVG. Age, extent of peripheral anterior synechia, surgical history (cataract, glaucoma, vitrectomy), and concurrent retinal cryotherapy were evaluated to determine factors influencing the surgical outcome. The main criterion for success was a postoperative intraocular pressure (IOP) of ≤21 mm Hg. Results: The cumulative probability of success was 67.0% at 1 year and 61.8% after 2 to 3 years. The surgical outcome was significantly better in patients without a previous vitrectomy (p = 0.03). Extensive preoperative peripheral anterior synechia was also a risk factor for surgical failure (p = 0.013). Conclusions: Trabeculectomy with MMC can effectively reduce the elevated IOP associated with NVG. The extent of peripheral anterior synechia and a history of vitrectomy are significant negative predictors of surgical outcome.