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PURPOSE To evaluate the safety of pupillary dilation in primary angle closure suspects (PACS) with concurrent visually significant cataract (VSC), to identify risk factors associated with elevated IOP, and to describe changes in anterior segment anatomy following pupillary dilation. DESIGN Prospective study. PARTICIPANTS Patients with PACS and VSC and no prior laser or intraocular surgery were recruited. VSC was defined as best correct visual acuity ≤ 20/40 due to cataract. METHODS Subjects were dilated with 0.5% tropicamide and 0.5% phenylephrine hydrochloride. A standardized eye examination, biometry, and swept-source anterior segment optical coherence tomography (SS-OCT) were performed prior to dilation. Intraocular pressure (IOP) and SS-OCT were repeated 1, 4, and 6 hours post-dilation (PDH1, PDH4 and PDH6). All parameters were compared between time points before and after dilation using paired t-test. Linear regression models were used to determine the risk factors associated with post-dilation IOP changes. MAIN OUTCOME MEASURES Change in IOP and SS-OCT parameters from baseline. RESULTS Seventy-eight eyes from 78 patients were included with 78, 66 and 12 patients completing the study at PDH1, PDH4 and PDH6, respectively. Mean IOP increased from 14.8 ± 2.6 mmHg at baseline to 15.5 ± 3.5 mmHg at PDH1 (P=0.03) and decreased to 14.9 ± 3.1 mmHg at PDH4 (P=0.09). Four (5.13%) and 3 (3.85%) patients had an increase in IOP ≥5 mmHg at PDH 1 and PDH4, respectively. Two (2.56%) and 1 (1.28%) had an increase in IOP ≥8 mmHg at PDH1 and PDH4, respectively. None developed acute primary angle closure during the observation period. Almost all anterior chamber parameters showed a significant increase after dilation at PDH1 and PDH4, except lens vault and iris volume, which decreased at PDH1 and PDH4 from baseline. Increase in anterior chamber depth was negatively associated with the level of IOP elevation following dilation (P<0.01). CONCLUSIONS Dilation of patients with PACS and VSC in this cohort appears to have a low risk for IOP spike. This may be associated with relaxation of the ciliary muscle leading to posterior displacement of the lens-iris diaphragm and deepening of the anterior chamber.