Atrial fibrillation and chronic kidney disease often co-exist and have shared risk factors. For this reason, the direction of the association between the two diseases is not clear. In a bidirectional Mendelian randomisation design, atrial fibrillation is causally related to a lower estimated glomerular filtration rate (eGFR) and a 6.1% higher risk of chronic kidney disease, while genetically predicted eGFR does not change the risk of atrial fibrillation.