Objective To evaluate whether the addition of phenylephrine to tropicamide produces any clinically significant change in pupil size during diagnostic eye examination. Methods Twenty healthy adults at the Washington University School of Medicine Eye Clinic were enrolled in this prospective, nonrandomized, crossover trial. Each had 3 dilating eye drop regimens administered to the left eye on separate days. Tropicamide (T) + proparacaine (PP) + phenylephrine (PE) (T+PP+PE) was considered the standard therapy, to which tropicamide alone (T alone) and tropicamide + proparacaine (T+PP) were compared against. Main outcome measures were postdilation pupil size and proportion of pupils able to achieve adequate clinical pupil dilation of >7 mm. Comparisons were made using Wilcoxon signed-ranked tests and McNemar's test. Results Mean postdilation pupil size was 7.94 ± 0.78 mm, 7.64 ± 0.78 mm, and 7.48 ± 0.77 mm for T+PP+PE, T+PP, and T alone, respectively. T+PP+PE was statistically superior to T+PP (p = 0.004) and T alone (p < 0.001) with respect to postdilation pupil size. The proportion of pupils able to achieve adequate pupil dilation of >7 mm was 90%, 80%, and 70% for T+PP+PE, T+PP, and T alone, respectively. No statistical difference was observed in each regimen's ability to achieve adequate pupil dilation of >7 mm (T+PP+PE and T+PP: p = 0.47; T+PP+PE and T alone: p = 0.13). Conclusion The addition of phenylephrine eye drops to tropicamide produced larger pupil dilation, but the magnitude of benefit was marginal and clinically insignificant in this young, healthy cohort. A single-dilating-agent regimen using tropicamide could be considered in routine clinical practice.