The longitudinal relaxation rate of protons is increased when subjects switch from breathing air to inhaling 100% oxygen. This effect when detected by MRI scanning is termed oxygen-enhanced MRI. It was found that this technique—in combination with measurements of perfusion—was capable of in vivo mapping of tumor hypoxia, distinguishing tumor subregions with low oxygen tension from well-oxygenated tumor tissue. Pimonidazole adduct formation immunofluorescence was used to validate these findings. Oxygen-enhanced MRI can be performed readily on conventional clinical scanners, so the technique has potential for rapid clinical translation. For details, see article by O’Connor and colleagues on page 787.