The effect of gadolinium-based contrast agents (GBCAs) on dynamic PET quantification was investigated. Simultaneous PET/MRI was performed in patients with biopsy confirmed invasive breast cancer. Subjects received an injection of a GBCA as part of standard clinical dynamic contrast enhanced breast MRI exam. SUVmean and SUVmax were calculated for the biopsy-proven breast cancer, aorta, liver, and benign fibroglandular tissue prior to and following GBCA injection. No significant difference in SUVmean or SUVmax was measured suggesting GBCAs are unlikely to affect PET quantification in combined breast PET/MRI.