Abstract Funding Acknowledgements Type of funding sources: None. Introduction Many patients with cardiac implantable electronic devices (CIED) need to undergo magnetic resonance imaging (MRI). However, a significant proportion has a CIED system in place that has not been classified "MRI-conditional" because of generators and leads from different brands ("mixed-brand" group), although the individual components per se are MRI-conditional. There is only limited data available concerning the outcome of these patients with "mixed-brand" CIED systems undergoing MRI. Purpose To analyse complications or adverse effects in "mixed-brand" CIED patients undergoing MRI, compared to patients with a fully "MRI-conditional" CIED system. Methods A retrospective single-centre study was performed, including patients undergoing MRI between January 2013 until May 2020. Short- and long-term outcomes were compared between both groups. We defined the primary endpoint as death or any adverse event necessitating hospitalization or CIED revision in association with the MRI examination. Secondary endpoints were the occurrence of any surrogate for beginning device or lead failure or patient discomfort during MRI. Results A total of 227 MRI examinations, including 10 thoracic MRIs, were carried out in 158 patients, with a range of 1-9 MRIs per patient. Mean age was 73 years and 52 (32.9%) patients were female. We identified 38 patients undergoing 54 procedures in the "mixed-brand" group and 89 patients undergoing 134 MRI procedures in the "MRI-conditional" group. In 31 patients undergoing 39 MRI examinations the MRI-conditionality could not be determined. "Mixed-brand" patients were older than MRI-conditional patients (mean 77 vs. 72 years, p = 0.003). The primary endpoint occurred in 0% in the "mixed-brand" group and in 2.2% in the "MRI conditional" group (p = 1.000). Complications were as follows: Two patients had first diagnosed atrial fibrillation directly associated with the procedure, of whom one additionally had a transient CIED dysfunction. No patient in the "mixed-brand" group and three patients (3.4%) in the "MRI conditional" group met the secondary endpoints (p = 0.554). In patients with undeterminable MRI conditionality the complication rate was similar (0% for both the primary and secondary endpoints). Conclusion The complication rate of CIED patients undergoing MRI was low. There was no signal for increased risk of adverse events in patients with a CIED without MRI certification due to mixed brands systems compared to patients with "MRI-conditional" CIED systems.