The overall thickness of the iris was classified as either medium or thick in 80% to 90% of all eyes. However, when the peripheral one third of the iris was considered alone, the proportion with medium or thick grades tended to be much less (50%–80%). The difference was not statistically significant between the overall iris thickness of cases and controls (χ2 test, cases versus control-1: P = 0.716 for superior, 0.677 for nasal, 0.535 for inferior, and 0.651 for temporal; cases versus control-2: P = 0.691 for superior, 0.645 for nasal, 0.488 for inferior, 0.535 for temporal). However, the thickness of the peripheral one third of the iris (basal iris thickness, as defined in the present study) tended to be higher in the cases in all quadrants than in the controls, although the differences in proportions of basal iris thickness grades between the cases and controls were statistically significant only in the superior quadrant (cases versus control-1: P = 0.008 for superior quadrant, 0.365 for nasal, 0.068 for inferior, and 0.932 for temporal; cases versus control-2: P = 0.009 for superior, 0.370 for nasal, 0.108 for inferior, and 0.990 for temporal).