To the Editor We read with interest the article by Conen et al¹ published in the April issue of JAMA Cardiology. In an analysis of the Women’s Health Study, the authors found an association between incident atrial fibrillation (AF) and cancer. While the study cohort was large, the incidences of AF and cancer were low, placing the findings at risk of a type II error. In addition, this association was not generalizable to men. Therefore, we sought to validate these findings in a larger real-world cohort among both sexes.

Using the Explorys platform,² an aggregated electronic database spanning inpatient and outpatient records from 26 major integrated health care systems across the United States, we identified 11 207 890 women and 9 003 530 men with characteristics similar to the population in the study by Conen et al¹ (ie, 45 years or older and free of major cardiovascular disease and cancer at baseline). These cohorts were examined for new-onset AF or cancer between June 2011 and May 2016 and analyzed using logistic regression.