Vessel size imaging is an emerging magnetic resonance imaging (MRI) technique which has been demonstrated to provide clinically relevant information about microvascular morphology. While previous studies of vessel size in humans relied on MRI contrast agents or hypercapnia-induced changes in blood oxygenation, the technique described here uses transient hyperoxia to alter the venous blood oxygenation. The experimental paradigm consisted of two 3-minute intervals of breathing 100% O-2 interleaved with three 2-minute intervals of breathing room air. Parametric maps of the mean venous vessel radius were calculated from changes in the blood oxygenation level dependent (BOLD) contrast which were measured using a combined spin-echo (SE) and gradient echo (GE) echo-planar imaging (EPI) sequence. The corresponding mean values in grey and white matter were r = 6.5 +/- 0.3 mu m and r = 6.2 +/- 0.3 mu m (n = 6). While the hypercapnia technique requires a specialised gas mixture containing a low concentration of CO2 (typically 5-6%), the hyperoxia technique presented here uses the inhalation of medical oxygen (100% O-2) which is routinely available in a clinical environment. Furthermore, 100% O-2 is generally better tolerated than low doses of CO2 which makes this technique particularly suitable for applications in critically ill patients. (C) 2011 Elsevier Inc. All rights reserved.