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To study the relationship between pterygium size (extension, width, total area) and corneal astigmatism in eyes with unilateral primary pterygium. Also to determine the critical size for surgery before the occurrence of a significant corneal astigmatism.This study was conducted on 77 eyes of 77 patients with unilateral primary pterygium. The extension and width were measured and the total area was calculated. Automated keratometry was used to determine corneal astigmatism.Pterygium extension ranged from 0.25 to 6.50 mm (mean, 2.0 ± 1.2 mm), width ranged from 1.50 to 10.0 mm (mean, 4.19 ± 1.5 mm), and total area ranged from 0.3 to 24.3 mm2 (mean, 5.0 ± 4.8 mm2). The mean value of corneal astigmatism was significantly (P < 0.0001) higher in pterygium-affected eyes (1.2 ± 0.9 D) than control eyes (0.6 ± 0.5 D). With-the-rule was the main type of pterygium-induced astigmatism (49.4%), followed by against-the-rule (36.4%) and oblique (14.3%). Pterygium extension had the best correlation (Pearson correlation coefficient r = 0.462, P < 0.001), followed by total area (r = 0.447, P < 0.002) and width (r = 0.348, P < 0.002). A stronger correlation was noted between pterygium size and the difference in corneal astigmatism between pterygium-affected eyes and control eyes. Pterygium induced 2 D of corneal astigmatism when its extension exceeded 2.2 mm, width exceeded 5 mm, or total area exceeded 6.25 mm2.Pterygium extension and total area have a stronger correlation with corneal astigmatism than does width. Surgical intervention is indicated when pterygium extension exceeded 2.2 mm, width exceeded 5 mm, or total area exceeded 6.25 mm2.