To evaluate the effect of cataract surgery on frequency doubling technology (FDT) perimetry in patients with co-existing cataract and glaucoma.In this consecutive prospective cohort study 27 patients with open angle glaucoma scheduled for cataract extraction alone or combined with trabeculectomy were enrolled. All patients underwent FDT threshold C-20 visual fields within 3 months before and 3 months after surgery. Changes in mean deviation (MD) and pattern standard deviation (PSD) were evaluated. Additionally, changes in best corrected logMAR visual acuity (VA), intraocular pressure (IOP), and number of glaucoma medications were also studied.22 patients completed the study. VA improved after surgery, from 0.47 (SD 0.19) to 0.12 (0.17) (p<0.001). The visual indexes changed after cataract extraction: MD improved (from -10.9 (SD 4.6) dB to -7.0 (4.6) dB; p<0.001) while PSD worsened (from 7.1 (SD 3.5) dB to 8.5 (3.8) dB; p = 0.001).In patients with co-existing cataract and glaucoma, examined with FDT, MD improved and PSD worsened after cataract surgery. Global indexes of FDT should be interpreted with caution in patients with glaucoma and cataracts.