Background: Commonly used strategies for cell delivery to the heart are intramyocardial injection and intracoronary (IC) infusion, both having their advantages and disadvantages. Therefore, alternative strategies, such as retrograde coronary venous infusion (RCVI), are explored. The aim of this confirmatory study was to compare cardiac cell retention between RCVI and IC infusion. As a secondary end point, the procedural safety of RCVI is assessed., Methods: Four weeks after myocardial infarction, 12 pigs were randomised to receive mesenchymal stromal cells, labelled with Indium-111, via RCVI (n=6) or IC infusion (n=6). Four hours after cell administration, nuclear imaging was performed to determine the number of cells retained in the heart both in vivo and ex vivo. Procedure-related safety measures were reported., Results: Cardiac cell retention is significantly lower after RCVI compared with IC infusion (in vivo: RCVI: median 2.89% vs IC: median 13.74%, p=0.002, ex vivo: RCVI: median 2.55% vs IC: median 39.40%, p=0.002). RCVI led to development of pericardial fluid and haematomas on the frontal wall of the heart in three cases. Coronary venous dissection after RCVI was seen in three pigs, of which one also developed pericardial fluid and a haematoma. IC infusion led to no flow in one pig., Conclusion: RCVI is significantly less efficient in delivering cells to the heart compared with IC infusion. RCVI led to more procedure-related safety issues than IC infusion, with multiple cases of venous dissection and development of haematomas and pericardial fluid collections., Competing Interests: Competing interests: WAG, MvdN, KN, JPGS, PAD and SAJC report grants from the Netherlands CardioVascular Research Initiative (CVON): the Dutch Heart Foundation, Dutch Federations of University Medical Centers, the Netherlands Organization for Health Research and Development, and the Royal Netherlands Academy of Sciences, during the conduct of the study. JPGS reports grants from Horizon2020 ERC-2016-COG EVICARE, grants from Technobeat, grants from the Project SMARTCARE-II of the BioMedicalMaterials institute, co-funded by the ZonMw‐TAS program, grants from the Dutch Ministry of Economic Affairs, Agriculture and Innovation, during the conduct of the study. WAG, FJvS and SAJC report non-financial support from Cook Regentec, 1102 Indiana Avenue Indianapolis, USA, during the conduct of the study. Catheters for RCVI were provided by Cook Regentec, 1102 Indiana Avenue Indianapolis, USA. On-site training with these catheters was facilitated by Cook Regentec. The authors did not receive payment from Cook Regentec to perform this study. Neither do the authors have stock options in Cook Regentec. Cook Regentec reviewed the manuscript prior to submission. BRvK, AET and JLMB declare no conflict of interest., (© Author(s) (or their employer(s)) 2019. Re-use permitted under CC BY. Published by BMJ.)