To evaluate the effect of phacoemulsification on trabeculectomy function.Retrospective case-control study.Forty-eight patients who underwent trabeculectomy surgery and had at least 2 years of follow up.Patients were classified into two groups: patients who had phacoemulsification subsequent to trabeculectomy (Trab_phaco, n = 18) and patients who were pseudophakic for greater than 6 months preceding their trabeculectomy (Phaco_trab, n = 30). Groups were matched for length of follow up of 2 years from time of trabeculectomy.The primary outcome measures were target intraocular pressure of criteria A, ≤12 mmHg; B, ≤15 mmHg; C, ≤18 mmHg with or without additional topical treatment. A separate measure for bleb function failure was also used; with failure defined as the need for additional topical antiglaucoma therapy or surgical intervention to achieve control of intraocular pressure.There was no significant difference in achieving each intraocular pressure criterion between groups (12 months, P = 1.0; 24 months, P = 0.330). In the first 12 months, significantly more trabeculectomies in the Trab_phaco group failed, requiring additional intervention to control the IOP (39%) compared with the Phaco_trab (10%) group (P = 0.028). Although this trend continued at 24 months, there were no significant differences in failure rates (P = 0.522).Phacoemulsification performed after trabeculectomy significantly increased rates of bleb failure in the following 12 months but not at 24 months.