We evaluated the retrobulbar response to a selective versus nonselective beta blocker in a subgroup of primary open-angle glaucoma patients (POAG) characterized by ocular vasospasm. Eleven patients who exhibited ocular vasospasm (i.e. a significant increase in ophthalmic artery blood flow velocity or a significant decrease in ophthalmic artery resistance index during hypercapnia) underwent medication washout for 4 weeks and were enrolled in a double-masked cross-over study (betaxolol versus timolol). Patients were evaluated for blood flow velocity of the retrobulbar vessels using color Doppler imaging, intraocular pressure, visual field sensitivity and contrast sensitivity at the beginning and end of each 4 week treatment period. Timolol treatment caused a significant reduction in IOP (p = .007), but no change in retrobulbar hemodynamics or visual function. After betaxolol treatment, resistance index fell significantly (p = .040) in the ophthalmic artery and increased significantly in both the central retinal (p = .003) and temporal posterior ciliary arteries (p = .030). Also following betaxolol treatment, contrast sensitivity improved significantly (p = .006), and a significant positive correlation was shown between change in contrast sensitivity and change in resistance index (r = .70; p = .015) of the ciliary arteries. POAG patients characterized by ocular vasospasm display a significant hemodynamic response to betaxolol, but not to timolol.