ABSTRACT Patients with pulmonary arterial hypertension (PAH) have reduced health related quality of life (HRQOL) and impaired exercise capacity. Despite fourteen approved therapies, most patients die within ten years. This grim fact underscores the need to develop interventions that improve HRQOL, particularly targeting mechanisms that are non-redundant to existing PAH therapies. Increasing physical activity is highly efficacious in PAH, resulting in six-minute walk distance (6MWD) and HRQOL improvement that often exceeds the effect of medications. Prior activity studies required inpatient rehabilitation, which is impractical, hard to sustain, and poorly scalable to a rare disease. Moreover, rehabilitation for PAH is not typically reimbursed by insurance in the United States, highlighting the need for alternatives to promote physical activity. This application builds on a recently-completed, NIH-funded pilot and feasibility trial of a mobile health (mHealth) intervention in PAH. In the pilot, subjects wore a Fitbit device and were randomized to either usual care or a fully automated “smart text” intervention. Text content was based on behavioral change theory and personalized to each subject. Texts were sent to the intervention arm 3 times/day with encouraging messages based on real-time activity data. Each subject had a personalized step count target which increased by 20% every 4 weeks. At the end of 12 weeks, the intervention arm took 1019 more steps per day than the control arm, an increase of 20% over baseline. We now hypothesize that increasing step counts will improve HRQOL in patients with PAH. We propose a randomized trial of smart texts versus usual care for 6 months. We will randomize 100 PAH patients to the mHealth intervention or usual care. All enrollment, monitoring, and data collection will occur remotely at Vanderbilt. We will enroll subjects across the United States, increasing generalizability. Our enrollment targets are feasible because we are supported by two large, existing PAH cohorts – the NIH-funded PVDOMICS Consortium and the Pulmonary Hypertension Association Registry. R61 Milestones (Year 1): IRB approval; establish DSMB; create REDCap database; Data Use Agreements; programming of text intervention; enrollment of first 10 subjects. In Aim 1 (primary endpoint), we will test the effect of a text-based mHealth intervention on HRQOL in PAH using the PAH-specific emPHasis-10 questionnaire. In Aim 2 (secondary endpoint), we will test the effect of an mHealth intervention on exercise capacity, measured by a highly feasible and reproducible supervised home-based 6MWD test. In an exploratory aim, we will examine the effect of the intervention on time to clinical worsening (composite of PAH therapy escalation, PAH hospitalization, and death) one year after randomization. This proposal will test a novel, highly scalable, and affordable mHealth intervention to improve clinically meaningful outcomes in patients with PAH.