Pulmonary arterial hypertension (PAH) is a disease that can eventually progress to right ventricular failure. Heart rate variability (HRV), including standard deviation of R-to-R intervals (SDNN), has been associated with increased mortality across different populations. The purpose of this study was to examine the association between HRV and disease severity in PAH. This was a prospective cohort study of adult incident PAH patients who underwent cardiac magnetic resonance imaging (CMR) at baseline and following 6 months. Study subjects recorded HRV using wearable chest actigraphy devices for at least 1 h every 2 weeks. The primary end point was correlation between HRV and right ventricular ejection fraction (RVEF) on CMR. Multivariable mixed effects regression models were used. A total of 20 subjects completed the study, predominantly female and White race, with severe PAH at the time of enrollment that improved significantly following 6 months of treatment. Linear relationships were observed between HRV (SDNN) and measures of PAH severity on CMR, including RVEF (SDNN coefficient 0.18,