Measurements of cerebral blood flow (CBF) using 15O-water-PET and ASL-MRI have been compared previously. Until now the two procedures were not performed simultaneously. Using a 3TMR-BrainPET, which integrates a BrainPET developed by Siemens as an insert in a Siemens 3T MAGNETOM MRI, we studied 15O-water-PET and ASL-MRI simultaneously in 10 healthy young men. In this way physiological and functional side conditions could be avoided which are possible in the separate PET/ASL comparisons published until now. Our initial results showed a similar global CBF of 50.1±7.2 ml/100 g/min and 51.9±7.1 ml/100 g/min with PET and ASL, respectively. While the CBF measured with ASL in white matter was lower than that of PET, the opposite result was obtained in gray matter. Direct comparisons in the single subjects showed local differences between ASL and PET as well as obvious distortions in ASL images. Although the global CBF between the two methods is similar, further investigations to improve the measurement of white matter perfusion in ASL are needed. © 2013 Elsevier B.V. All rights reserved.