Ocular lesions, including cotton wool spots and retinal hemorrhage, are a common feature of HIV infection and acquired immunodeficiency syndrome (AIDS). The aetiology of these apparently vasoocclusive phenomena in HIV related retinopathy is not well understood. Several hypotheses including infectious damage of the retinal vasculature and altered retinal hemodynamics have been postulated. The latter would be compatible with the theory that the retina of HIV patients is hypoxic. However, direct measurement of oxygen tension in the retina is not possible and indirect methods have to be employed. The study objective was to investigate the reactivity in retinal blood flow to 100% oxygen breathing in patients with HIV.