Glaucoma filtration surgery (trabeculectomy) is an effective glaucoma treatment method that provides significant intraocular pressure (IOP) reduction. Indications for this method are in cases where other forms of therapy, like medicines or lasers, have failed, other forms of therapy are not suitable (compliance or side-effects are a problem), in cases where a target pressure is required to prevent clinically significant disease progression that cannot be reached with topical medications and/or laser and in cases that have such advanced glaucoma and high IOP at presentation that other forms of treatment are unlikely to be successful. The aim of this study was to evaluate the effect of glaucoma surgery on the structural and functional eye parameters at different stages of glaucoma. A total of 96 eyes of 96 patients (only one eye from each patient) with different stages of glaucoma (stages 2 to 4) who were undergoing trabeculectomy were recruited. Quadrant retinal nerve fibre layer RNFL) thickness (33 patients), cup/disc vertical and horizontal ratio (36 patients) and MD of visual fields (27 patients) were analysed up to one week before and 1 month after the successful surgery. The results show that the MD value was slightly improved in 50%, 85.7%, and 71.4% of patients with glaucoma stages 2, 3, and 4, respectively. The mean RNFL of all four optic nerve head quadrants increased slightly after the surgery for patients with glaucoma stage 2; in contrast, a decrease in the mean RNFL values for all four quadrants was observed for patients with glaucoma stage 4. Statistically significant changes in the mean values for the optic nerve horizontal c/d ratio after glaucoma surgery were observed (p = 0.033) in contrast to the vertical c/d values (p = 0.77). In total, improvement of the horizontal and vertical c/d ratio was detected in 61.1% and 55.6% of the glaucoma patients, respectively. Although the observed changes were statistically insignificant, the positive influence of glaucoma surgery on the structural and functional eye parameters was more pronounced in moderate stages than in advanced or severe stages of glaucoma.