Trabeculectomy, despite producing an effective reduction in intraocular pressure, may not prevent continued visual field loss. This may be because of the presence of other factors in the pathogenesis of glaucoma. Vascular factors have been suggested as being particularly important. To study the effect of trabeculectomy on ocular blood flow the technique of ocular pulse analysis has been used to derive a measure of pulsatile ocular blood flow in 17 patients (average age 65.6 (SD 1.8) years) undergoing trabeculectomy. A significant increase in pulsatile ocular blood flow of 29% was observed in the group as a whole in the standing position following operation but in some individuals blood flow changed only slightly despite a large reduction in intraocular pressure. The significance of these findings in relation to the prognosis of visual field preservation following trabeculectomy is discussed.