Pulmonary Arterial Hypertension (PAH) is a rare incurable condition affecting both the cardiac and respiratory systems. Patients living with PAH face the burden of both intensive medication regimens and debilitating disease symptoms. This study’s primary aim was to identify patients’ medication-taking behaviours and beliefs using a framework derived from the extended health belief model (EHBM), and to use this information to deliver personalised interventions to improve medication-taking behaviours. A mixed-methodology longitudinal study design recorded patients’ parameters over a 12-month period. Thirteen participants from Northern Ireland completed the study. The results showed that the level of high-adherence to PAH medicines, as assessed using the MARS questionnaire was 80%, but this value differed when assessed via pill counting and interview data. There was a trend to improvement in observed and predicted medication adherence over the study duration. Participants’ beliefs showed a non-statistical increase in the specific-necessity beliefs and a reduction in general-overuse belief. This study added to the EHBM new constructs of trust and support in being able to better predict nonadherent behaviours. Key medication-taking themes were self-confidence, perceived ranking of medicines, uncertainty and knowledge. This study developed important learning that can be applied to future research on behavioural health studies. / Heart Trust Fund; Actelion Pharmaceuticals