AIMS--The laser Doppler technique was used to compare the capillary blood speed measured at localised sites of the optic nerve head in stable, untreated ocular hypertensive patients with that measured in healthy normal subjects. The stereophotogrammetric technique was also used to measure the retinal nerve fibre layer thickness at the disc margin in the eyes of the patients. METHODS--Doppler broadening measurements were made at superior and inferior temporal disc sites in 18 eyes of 10 ocular hypertensive patients and in 12 eyes of seven age and sex-matched normal subjects. RESULTS--On average, Doppler broadening and, hence, capillary blood speed were significantly higher (p = 0.018) in the patients than in the normal subjects. The largest values of Doppler broadening in the patients were measured at sites adjacent to the thinnest retinal nerve fibre layer. Linear regression analysis showed a significant inverse relation (p = 0.0004) between Doppler broadening and nerve fibre layer thickness in left eyes, and a nearly significant relation (p = 0.06) in right eyes. At temporal sites of the optic nerve head there is a compensatory relation between a thinning nerve fibre layer and a locally increasing blood supply to the optic nerve head. CONCLUSION--Together with previous observations of fluorescein filling defects in similar patients, these results indicate that there is spatial heterogeneity of blood flow in the optic nerve head in stable, untreated ocular hypertensive patients.