The relationship between blood flow velocities in retrobulbar vessels and blood flow at the optic nerve in glaucoma patients was assessed in a prospective study. The Heidelberg retina flowmeter (HRF) was used to assess optic nerve head blood flow in 13 open-angle glaucoma patients. In the same patients, color Doppler imaging (CDI) measurements were obtained from the ophthalmic artery, the central retinal artery and the posterior ciliary arteries. Using data for one randomly selected eye per subject, correlations between HRF recordings and CDI measurements were evaluated by means of Spearman’s rank correlation factor. All three HRF parameters correlated with CDI measurements obtained from retrobulbar vessels. The most marked correlations were those of the HRF parameter ‘volume’ with the end-diastolic velocity in the ophthalmic artery and the medial posterior ciliary artery (R = 0.79, p = 0.0012 and R = 0.81, p = 0.0007, respectively), and the peak systolic velocity in the lateral posterior ciliary artery (R = 0.82, p = 0.0006). The present study suggests that glaucoma patients with altered blood flow in retrobulbar vessels are likely to show an alteration in optic nerve blood flow as measured with the HRF.