Purpose: To measure the rate of complications from chronic hypotony following trabeculectomy and clarify the definition of postoperative hypotony. Materials and methods: In this retrospective case-control study, the rate of complications was compared between 34 eyes with chronic hypotony and 34 eyes without hypotony. Chronic hypotony was defined as those eyes with an intraocular pressure (IOP) of less than 6 mm Hg on two consecutive clinic visits at least 3 months after trabeculectomy. Cases were identified from a database of two glaucoma surgeons between 2010 and 2013. Outcomes measured included visual acuity, presence of choroidal effusion, hypotensive maculopathy and cataract development/progression. Factors associated with the development of hypotony were considered using analysis of variance (ANOVA) multivariate regression. Results: Maculopathy was seen in 23.5% of hypotony eyes but not in controls (p < 0.01). No significant difference in the rate of choroidal effusion or cataract was documented between groups. Control eyes were more likely to remain complication free (58.8 vs 32.4%, p < 0.03). Spontaneous recovery from hypotony occurred in 32.4% of hypotony eyes. Conclusion: Sight threatening complications occur more frequently in eyes with chronic hypotony following glaucoma surgery. However, not all eyes with chronic hypotony develop sight threatening complications. A definition of hypotony that combines IOP criteria with the presence of structural and/or functional changes is recommended.