Abstract Background ¹⁸F-Fluoride uptake denotes calcification activity in aortic stenosis and atherosclerosis. While PET/MR has several advantages over PET/CT, attenuation correction of PET/MR data is challenging, limiting cardiovascular application. We compared PET/MR and PET/CT assessments of ¹⁸F-fluoride uptake in the aortic valve and coronary arteries. Methods and results 18 patients with aortic stenosis or recent myocardial infarction underwent ¹⁸F-fluoride PET/CT followed immediately by PET/MR. Valve and coronary ¹⁸F-fluoride uptake were evaluated independently. Both standard (Dixon) and novel radial GRE) MR attenuation correction (AC) maps were validated against PET/CT with results expressed as tissue-to-background ratios (TBRs). Visually, aortic valve ¹⁸F-fluoride uptake was similar on PET/CT and PET/MR. TBR_MAX values were comparable with radial GRE AC (PET/CT 1.55±0.33 vs. PET/MR 1.58 ± 0.34, P = 0.66; 95% limits of agreement − 27% to + 25%) but performed less well with Dixon AC (1.38 ± 0.44, P = 0.06; bias (−)14%; 95% limits of agreement − 25% to + 53%). In native coronaries, ¹⁸F-fluoride uptake was similar on PET/MR to PET/CT regardless of AC approach. PET/MR identified 28/29 plaques identified on PET/CT; however, stents caused artifact on PET/MR making assessment of ¹⁸F-fluoride uptake challenging. Conclusion Cardiovascular PET/MR demonstrates good visual and quantitative agreement with PET/CT. However, PET/MR is hampered by stent-related artifacts currently limiting clinical application.