Understanding the experiences of people who have postural orthostatic tachycardia syndrome (POTS) and the health professionals who care for them: a grounded theory study. Background: Postural orthostatic tachycardia syndrome (POTS) is an under recognised, predominantly female condition. Awareness of this condition has increased due its association with Long-COVID. Understanding experiences informs areas for future study. Research into the experience of POTS is emerging, with no currently published studies including the experiences of health professionals. Aim: This study aimed to understand experiences of POTS, its challenges, and aspects of care from the perspectives of people with the condition and health professionals who look after them. Design: A grounded theory approach was taken to develop a conceptual framework of the experience of POTS conceptualised through a critical realist lens. Setting: This United Kingdom based study included people with POTS from three national health care centres and a diverse range of health professionals. Participants: Theoretical and purposive sampling recruited people with POTS, and snowball sampling recruited health professionals through a United Kingdom charity and arrhythmia nursing network. Health professionals were interviewed from a range of professions including nurses, secondary care consultants, primary care general practitioners, psychologists, physiotherapists, and an occupational therapist. Method: Between September 2021 and September 2022 in-depth online semi-structured interviews were undertaken. Data were thematically analysed. Corbin and Strauss' Grounded Theory methods of constant comparison, memos, and a reflexive journal were used. Analysis moved from first open coding of health professional themes to more focused axial coding, data, and category linking. The study patient and public involvement group informed thematic development and theoretical sampling. The final themes were conceptualised through the critical realism's three layers of experience. Results: Forty-four participants were included, 19 people with POTS and 25 health professionals. Three core themes were found: 1) A challenging condition, 2) POTS healthcare provision - services by accident not design, and 3) The need to validate experiences. Health professionals experienced challenges in their clinical selfefficacy. POTS care was driven by individual human agency and interpersonal relationships. People with POTS took individual responsibility for their self-management but were impacted by gender inequalities, a medical model prioritising guidelines, and a poor evidence-base. Conclusion: This is the first qualitative study to explore both people with POTS and health professional experiences of looking after people with this condition. Findings identified: 1) a need for empowering both people with POTS and health professionals through shared care and decision making, 2) the requirement for a system wide