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To examine refractive outcomes after phacoemulsification in eyes with prior trabeculectomy.Retrospective observational case-control studyComparison of eyes of glaucoma patients undergoing cataract surgery after trabeculectomy (study group) with a matched group with medically controlled glaucoma (control group). Laser interferometry was used to obtain ocular biometry. We measured the difference between the expected and actual postoperative refraction using third-generation and fourth-generation intraocular lens (IOL) prediction formulae (Haigis, Holladay 2, Hoffer Q, and SRK-T). A residual difference of >1.0 D of hyperopia or myopia was considered a “refractive surprise.”In total, 86 eyes (85 patients) were included, including 23 eyes (22 patients) in the study group and 63 eyes (63 patients) in the control group. The mean follow-up was 12.2±4.1 months. Eyes (n=13) with trabeculectomy and a preoperative intraocular pressure (IOP)≤9 mm Hg had significantly more large myopic surprises than the control group for all IOL formulae (P=0.015 Haigis, P=0.003 Holladay 2, P=0.004 Hoffer Q, P=0.003 SRK-T). Eyes (n=10) with trabeculectomy and preoperative IOP>9 mm Hg, however, did not have significantly more myopic errors than the control (P>0.05, all formulae). An “IOP spike” defined as a >50% rise in IOP from baseline within 1 month of cataract surgery in the subgroup with preoperative IOP≤9 mm Hg (n=8) was associated with increased risk of large myopic surprise (3/8 subset vs. 1/63 control eyes for all formulae; P=0.004 Haigis, P=0.004 Holladay 2, P=0.001 Hoffer Q, P=0.004 SRK-T) as well as for large myopic and hyperopic surprises overall (4/8 subset vs. ≤2/63 depending upon formulae; all P≤0.001).Low posttrabeculectomy IOP (≤9 mm Hg) is a risk factor for significant myopic surprise when undergoing subsequent cataract surgery despite using laser interferometry to measure ocular biometry and later generation formulae to determine IOL power. In addition, an IOP spike was associated with a 50% risk for large refractive surprise in this low IOP group.