PURPOSE: To determine whether the diameter of Bruch's membrane opening (BMO) can distinguish mild papilledema from pseudopapilledema using optical coherence tomography (OCT). METHODS: The medical records of pediatric patients with pseudopapilledma due to optic nerve head (ONH) drusen, patients with papilledema, and normal control subjects were retrospectively reviewed. All eyes underwent OCT imaging of the BMO and retinal nerve fiber layer (RNFL). Transverse horizontal diameter of the BMO and papillary height were measured. Mean BMO, papillary height, and RNFL were compared and receiver operating characteristic (ROC) curves were used to calculate the area under the curve (AUC) and determine BMO and RNFL cut-offs for papilledema and pseudopapilledema. RESULTS: A total of 90 eyes of 90 subjects were included: 58 with pseudopapilledema, 19 with papilledema, and 13 controls. In eyes with papilledema, mean BMO, papillary height, and RNFL decreased as papilledema resolved (1893.8 vs 1582.2 [P = 0.0003], 193.0 vs 108.9 [P < 0.0001], 893.3 vs 695.5 [P = 0.0007], resp.). Eyes with mild papilledema had greater mean BMO and RNFL than those with pseudopapilledema and controls (1893.8 vs 1541.9 vs 1628.8 [P < 0.0001, P = 0.0265] and 193.0 vs 108.7 vs 104.1 [P < 0.0001, P < 0.0001], resp.). Papillary height in mild papilledema was similar to pseudopapilledema but greater than controls (893.3 vs 863.2 vs 593.5 [P = 0.47 and P = 0.0001], resp.). ROC showed good diagnostic discrimination for BMO (AUC = 0.81; 95% CI, 0.70-0.92) and RNFL (AUC = 0.96; 95% CI, 0.93-1.0) in distinguishing mild papilledema from pseudopapilledema. CONCLUSIONS: The horizontal transverse diameter of BMO is enlarged in eyes with mild papilledema and narrows as papilledema resolves. BMO and RNFL can be used together to help distinguish mild papilledema from pseudopapilledema in children.