Using the laser Doppler technique we measured finger blood flow velocity in 25 untreated primary open-angle glaucoma (POAG), 22 untreated low-tension glaucoma (LTG) and 19 age-matched normal subjects. Four blood flow measurements were recorded: baseline flow, after immersion in warm water for 2 min (40 degrees C maximum flow), after 10 s exposure to cold water (4 degrees C minimum flow) and time to recover to baseline flow after cold immersion. Significant differences between the LTG group and both the normal and POAG groups were found in a greater maximum flow (p = 0.03 and p = 0.01, respectively), a lower minimum flow (p = 0.04 and p = 0.03, respectively) and a longer recovery time to baseline flow (p = 0. 0001 for both the normal and POAG groups). A longer recovery time to baseline flow (p = 0.008) in POAG was the only difference between the normal and POAG groups. Both glaucoma groups had more systemic vascular disease than normal, and there were more smokers in the POAG group than normals. Subgroup assessment performed with migraine, vascular disease and smoking subjects excluded from the analysis did not alter the overall results. We conclude that systemic vasospasm is a strong feature of low-tension glaucoma.