Self-management support tools using technology may improve adherence to hypertension treatment. There is a need for user-friendly tools facilitating patients' understanding of the interconnections between blood pressure, wellbeing and lifestyle. This study aimed to examine comprehension, comprehensiveness and relevance of items, and further to evaluate the usability and reliability of an interactive hypertension-specific mobile phone self-report system. Areas important in supporting self-management and candidate items were derived from five focus group interviews with patients and healthcare professionals (n = 27), supplemented by a literature review. Items and response formats were drafted to meet specifications for mobile phone administration and were integrated into a mobile phone data-capture system. Content validity and usability were assessed iteratively in four rounds of cognitive interviews with patients (n = 21) and healthcare professionals (n = 4). Reliability was examined using a test-retest. Focus group analyses yielded six areas covered by 16 items. The cognitive interviews showed satisfactory item comprehension, relevance and coverage; however, one item was added. The mobile phone self-report system was reliable and perceived easy to use. The mobile phone self-report system appears efficiently to capture information relevant in patients' self-management of hypertension. Future studies need to evaluate the effectiveness of this tool in improving self-management of hypertension in clinical practice.