PURPOSE. We studied the association between dynamic iris changes and the spectrum of primary angle closure disease (PACD), using the anterior segment optical coherence tomography (ASOCT). METHODS. Eligible primary angle closure (PAC), primary angle closure glaucoma (PACG), age, and sex comparable primary angle closure suspects (PACS) and normal subjects from the 5-year follow-up of the Handan Eye Study underwent ASOCT testing in dark and light conditions. The right eye of each subject was analyzed and biometric parameters including iris cross-sectional area (IA), lens vault (LV), pupil diameter (PD), and centroid-to-centroid distance (CCD) were calculated using the Zhongshan Angle Assessment Program. RESULTS. Totals of 31 PACS, 31 PAC/PACG, and 31 normal eyes were eligible for analysis. Loss of IA per mm PD increase in the dark compared to light was 0.18 mm in PACS, 0.13 mm in PAC/PACG, and 0.24 mm in normal (P = 0.015 between groups) groups. Diagnoses of normal (P = 0.001) and a smaller PD in light (P = 0.003) were statistically significant determinants of a larger IA loss per mm PD increase in the dark compared to light. Logistic regression analysis showed that LV (P = 0.002) and IA loss per mm PD increase (P = 0.017) were risk factors for an occludable angle. CONCLUSIONS. Significant differences in iris behavior in the dark compared to light in PACS, PACD, and normal eyes add to the evidence that dynamic iris change has a role in the pathogenesis of PAC in a rural Chinese population.