ABSTRACT Adolescents with pulmonary arterial hypertension (PAH) have impaired exercise capacity and reduced quality of life. Current therapies only modestly improve exercise capacity and are often prohibitively expensive, highlighting the need for additional, cost-effective interventions. Multiple studies have demonstrated that increasing physical activity is highly efficacious in adult PAH, resulting in six-minute walk distance (6MWD) improvement that exceeds the effect of medications. However, protocols require inpatient rehabilitation, making them impractical and poorly scalable to the adolescent PAH population. Moreover, cardiopulmonary rehabilitation is poorly reimbursed, and facilities are not easily available or accommodating of adolescents. Recently, in adults, we completed the first mobile health (mHealth) intervention in the PAH population, demonstrating the effectiveness of linking real time activity tracking with a “smart” behavioral coaching intervention, using personalized, encouraging content based in behavioral change theory (e.g. feedback loops and habit formation). After 12 weeks, the intervention arm took 1019 more steps per day than the control arm (adjusted for age, sex, functional class, and baseline step counts), with <5% dropout. In secondary analyses, we observed significant improvements in quality of life (QOL) and right ventricular function. But, adolescents may respond differently to remote real time activity monitoring and behavioral coaching, demanding optimization of our approaches. We hypothesize that a mHealth intervention specifically designed for the needs of an adolescent population with PAH is feasible and will increase daily step counts. We propose a randomized pilot trial of smart texts versus usual care for 12 weeks using personalized, adaptive step count targets and text messages that leverage age-appropriate behavioral psychology. Using a fully remote study without in-person visits, 50 participants will wear a Fitbit device and be randomized 1:1 to the mHealth intervention or usual care. Aim 1 will adapt our successful PAH-focused mHealth approach to the developmental needs and interests of adolescents. Participant, caregiver, and community feedback will be used to optimize our resource of smart text messages for study communication. In Aim 2, we will test the feasibility of a mHealth intervention to increase step counts in adolescents with PAH. Secondary endpoints will include measures of QOL for the adolescent and caregiver(s), activity duration and intensity, right ventricular function, as well as fidelity of text and data transmission. The goals of this proposal are to 1) develop and refine an adolescent-focused mHealth intervention for PAH and 2) obtain the feasibility and preliminary data needed to conduct a large, multi-center R01-funded trial across the U.S. with our Pediatric Network collaborators, which will be submitted for consideration during the last year of this award.