As currently implemented, magnetic resonance imaging (MRI) relies on the protons of water molecules in tissue to provide the NMR signal. Protons are, however, notoriously difficult to image in some biological environments of interest, notably the lungs1 and lipid bilayer membranes such as those in the brain2. Here we show that 129Xe gas can be used for high-resolution MRI when the nuclear-spin polarization of the atoms is increased by laser optical pumping and spin exchange3–6. This process produces hyperpolarized 129Xe, in which the magnetization is enhanced by a factor of about 105. By introducing hyperpolarized 129Xe into mouse lungs we have obtained images of the lung gas space with a speed and a resolution better than those available from proton MRI1,7 or emission tomography8,9. As xenon (a safe general anaesthetic) is rapidly and safely trans-ferred from the lungs to blood and thence to other tissues8,9, where it is concentrated in lipid10–15 and protein13,15–18 components, images of the circulatory system, the brain and other vital organs can also be obtained. Because the magnetic behaviour of 129Xe is very sensi-tive to its environment, and is different from that of 1H 2 O, MRI using hyperpolarized 129Xe should involve distinct and sensitive mechanisms for tissue contrast.