The velocity of blood flow in the ophthalmic artery was measured with a transcranial Doppler ultrasound (2 MHz). Sixty patients with chronic open angle glaucoma (COAG), 42 patients with normal tension glaucoma (NTG), and 35 normals were studied. Peak flow velocity (peak), mean-enveloped velocity (mean) and diastolic velocity were compared in the three groups. The COAG patients and the NTG patients showed a significant reduction of all three Doppler flow velocities compared with the normals (COAG vs normal, p = 0.013, 0.019, and 0.009; NTG vs normal, p = < 0.001, < 0.001, and < 0.001 for peak, mean and diastolic flow velocity respectively). The NTG patients also had a significant slower mean flow velocity than the COAG patients (p = 0.016), but not for peak and diastolic flow velocities (p = 0.060 and 0.052 respectively). Based on ophthalmic flow velocity values, many NTG patients and a few COAG patients had slower flow velocities than the normals. The significance of these haemodynamic differences is not yet known.