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Purpose: To evaluate iris volume before and after pupil dilation using swept‐source anterior segment optical coherence tomography (SS‐ASOCT) and investigate the associated factors of iris volume and iris volume change after pupil dilation in patients with type 2 diabetes mellitus (T2DM). Methods: A cross‐sectional study was conducted in Zhongshan Ophthalmic Center among T2DM registered patients in the community of Guangzhou, China. Anterior chamber volume (ACV), iris volume, anterior chamber depth (ACD), angle opening distance at 500 μm (AOD 500) and pupil diameter were estimated using SS‐ASOCT (CASIA; Tomey, Nagoya, Japan). Venous blood was taken for the measurement of glycosylated haemoglobin A1c (HbAlc). All biometric measurements were performed before and after pharmacologic pupil dilation. Results: A total of 117 subjects were included in the analysis. The mean age (±SD) was 64.96 ± 7.75 years, and 62.4% were females. After pupil dilation, iris volume decreased in all eyes. Shorter duration of diabetes (p = 0.035), longer axial length (p < 0.001) and smaller pupil diameter (p < 0.001) were associated with larger iris volume. The change in iris volume per millimetre change in pupil diameter was 1.35mm3/mm. Smaller baseline iris volume (p = 0.002) and higher HbA1c level (p = 0.010) were associated with smaller change in iris volume per millimetre change in pupil diameter, after adjusting for other factors. Conclusion: Iris volume can be estimated by SS‐ASOCT. Diabetic duration was associated with static anatomy of iris volume, while HbA1c level indicated the dynamic response of iris volume. It is likely that diabetic duration and HbA1c level could affect the macroscopic and microscopic composition of the iris in the diabetic population. [ABSTRACT FROM AUTHOR]