Larsson SC, Lee WH, Burgess S, et al. - Researchers used a two-sample Mendelian randomization (MR) design to ascertain if there exists a causal link of plasma cortisol with atrial fibrillation (AF). In the CORtisol NETwork consortium (12 597 participants), three genetic variants in the SERPINA1/SERPINA6 locus and functionally related to plasma cortisol were detected. A genome-wide association study (GWAS) meta-analysis of six studies (60 620 AF cases and 970,216 non-cases) and the FinnGen consortium (17,325 AF cases and 97,214 non-cases) were used to acquire summary-level GWAS data for the links between the cortisol-related variants and AF. As per evidence generated in the MR study, there exists a positive link between plasma cortisol and AF risk, possibly mediated via systolic blood pressure and waist circumference.