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To propose a grading system of pterygium severity based on corneal higher-order irregularity (HOI) and to evaluate postoperative changes in corneal irregularity of the graded pterygia.In 268 eyes of 226 patients undergoing excision surgery of primary nasal pterygium, Placido corneal topography images were taken before and until 6 months after surgery. The best-corrected visual acuity (BCVA) and pterygium size with respect to the corneal diameter were also measured. HOI components within 1.0-, 3.0-, and 5.0-mm diameters were obtained using Fourier harmonic analysis of corneal topography data. With threshold levels when the HOIs for 3 diameters steeply increased, 4 levels of grading from 0 (mild) to 3 (severe) were determined. Associations of the grades with the preoperative surface regularity index (SRI), irregular components (IRRs) obtained from the 3.0-mm diameter Fourier analysis, and BCVA were examined. Time-course changes in the indexes after surgery were evaluated for each grade.There were 62, 65, 41, and 100 eyes in grades 0 to 3, respectively. Preoperatively, the SRI in grade 3 was significantly higher than in the other grades (P < 0.001), and the IRR and BCVA varied with the grade (P < 0.001). Between 1 and 6 months postoperatively, grade 3 pterygia showed significant changes in the SRI and IRR (P = 0.013 and 0.017, respectively), whereas there was no change after 3 months postoperatively in the SRI, IRR, and BCVA for all other grades.The proposed grading effectively classified the severity of pterygia and evaluated postoperative restoration of corneal irregularity. Using corneal topographic data allowed for objective evaluation of pterygium severity.