A new MR approach has been proposed to obtain simultaneous measurements of blood oxygen saturation (SO2) and hematocrit (Hct) by measuring and combining blood MR relaxation times. Although the first results were encouraging, the method has not been properly validated. In this study, we tested this approach in 10 volunteers subjected to gas challenges with the intent to modify SO2 while keeping Hct constant. The method was also tested in 10 Moyamoya patients and compared to photometric analysis. Results suggest that reliable MR estimates of both SO2 and Hct can be obtained in vivo.