Purpose: To compare funduscopic and confocal scanning vertical cup-disc ratio (VCDR) assessments and their respective predictive value for estimating functional glaucomatous damage. Methods: Data from a single eye of open angle glaucoma patients from the Leuven Eye Study were included: age, gender, intra-ocular pressure, visual acuity, refractive error, visual field mean deviation and pattern standard deviation, funduscopic and HRT III VCDRs as well as mean retinal nerve fibre layer thickness. Non-parametric tests to compare differences within and between diagnostic groups were used, and receiver-operating characteristic curves as well as Bland-Altman plots constructed. Results: Three hundred and one eyes of 301 subjects with primary open angle glaucoma (POAG) and normal tension glaucoma (NTG) were included. The average VCDR assessed with HRT III was significantly smaller than the funduscopic measurement (0.69 ± 0.16 vs. 0.81 ± 0.14, respectively; p < 0.001). The predictive value of both measurement techniques did not differ in NTG patients, but the funduscopic estimate yielded a significantly larger predictive power in patients with severe POAG. Conclusion: Funduscopic and confocal scanner estimates of VCDR differ significantly and should not be used interchangeably. In POAG patients with severe glaucoma, a subjective VCDR predicts functional glaucomatous damage significantly better.