Zero-Echo-Time (ZTE) MRI for attenuation correction (AC) in hybrid PET/MRI-systems is a promising method. This study aims to examine reproducibility between 15O-water PET and simultaneously acquired Arterial Spin Labelling (ASL) using ZTE-MRI AC and the reproducibility between to subsequent ASL measurements. Measurements were performed on six subjects on an integrated 3.0 T PET/MRI-system. Regional cerebral blood flow (CBF) values from standard flow territories were compared. ASL showed good correlation with 15O-water PET, presenting another advantage of ZTE-MRI AC. A significant decrease between ASL measurements was detected, which may be important to consider when designing PET/MRI-studies.