Objective <p>To investigate the value of a circumference-based quantitative analysis of iris angiography (IA) in evaluating iris vascular changes across different stages of diabetic retinopathy (DR).</p> Methods <p>This observational study included 66 healthy subjects and 185 patients with diabetes, classified as no apparent retinopathy, non-proliferative diabetic retinopathy (NPDR), and proliferative diabetic retinopathy (PDR). All participants underwent ultrawide-field fundus fluorescein angiography and IA, including iris fluorescein angiography and iris indocyanine green angiography. A custom-built software was used to quantify fluorescein leakage at the pupillary margin by measuring leakage time (LT) and circumferential leakage range (LR, degrees). Comparisons among groups were performed using one-way analysis of variance.</p> Results <p>No fluorescein leakage was observed at the pupillary margin in healthy subjects aged 20–39 years, whereas mild and transient leakage was detected in older healthy patients. In patients with DR, LT was significantly shorter in the PDR group than in the no-retinopathy and NPDR groups (25.67 ± 5.03&#xa0;s vs. 33.14 ± 3.03&#xa0;s and 32.45 ± 5.17&#xa0;s, respectively; all <i>P</i> &lt; 0.001). The LR increased with DR severity, measuring 21.21 ± 30.06° in the no-retinopathy group, 62.48 ± 42.17° in the NPDR group, and 141.31 ± 73.61° in the PDR group (all <i>P</i> &lt; 0.001). LR values in the NPDR and PDR groups were significantly greater than those in age-matched healthy subjects.</p> Conclusion <p>Circumference-based quantitative analysis of IA enables quantitative assessment of pupillary-margin fluorescein leakage and was associated with DR severity. This method may provide additional information for evaluating anterior segment vascular involvement in patients with diabetic retinopathy, particularly when posterior segment examination is limited.</p> Trial registration <p>Chinese Clinical Trial Registry (ChiCTR2400081639), registered on Mar 7, 2024.</p>