Dosimetry-guided treatment planning in selective internal radiation therapy relies on accurate and reproducible measurement of administered activity. This 4-center, 5-PET-device study compared the manufacturer-declared ⁹⁰ Y activity in vials with quantitative ⁹⁰ Y PET/CT assessment of the same vials. We compared ⁹⁰ Y PET-measured activity (A _PET ) for 56 ⁹⁰ Y-labeled glass and 18 ⁹⁰ Y-labeled resin microsphere vials with the calibrated activity specified by the manufacturer (A _M ). Additionally, the same analysis was performed for 4 ⁹⁰ Y-chloride vials. The mean A _PET /A _M ratio was 0.79 ± 0.04 (range, 0.71-0.89) for glass microspheres and 1.15 ± 0.06 (range, 1.05-1.25) for resin microspheres. The mean A _PET /A _M ratio for ⁹⁰ Y-chloride vials was 1.00 ± 0.04 (range, 0.96-1.06). Thus, we found an average difference of 46% between glass and resin microsphere activity calibrations, whereas close agreement was found for chloride solutions. We expect that the reported discrepancies will promote further investigations to establish reliable and accurate patient dosimetry and dose-effect assessments.