OBJECTIVE To determine the effect of trabeculectomy on cataract formation or progression in patients with chronic glaucoma. METHODS This controlled clinical trial was performed on patients over 50 years of age with glaucoma who were referred to Imam Hossein Hospital, Tehran, Iran, from 2006 to 2007. Trabeculectomy was indicated only in one eye while the fellow eye had wellcontrolled intraocular pressure with medication(s). The fellow eyes served as controls. Lens opacity was evaluated using three criteria: visual acuity (VA), and Lens Opacification Classification System III (LOCS III) photographs and scores. RESULTS Overall 82 eyes of 41 patients including 53.7% male and 46.3% female subjects with mean age of 62.5±9.3 (range 50-75) years were evaluated. Cataract progression in operated eyes was statistically significant according to VA (P=0.02), LOCS III photographs (P=0.05) and LOCS III scores (P=0.01). However, compared to fellow control eyes, cataract progression was significant according to VA (P=0.023) and LOCS III scores (P=0.057) but not based on LOCS III photographs. Mean VA reduction was 2 Snellen lines in operated eyes; there were 3 cases of cataract formation or progression without reduced VA. CONCLUSIONS Cataracts seem to progress following trabeculectomy; therefore it might be advisable to perform a combined procedure in older patients with moderate lens opacities.