Although patients are often absent in discourses on telemedicine, many telemonitoring applications constitute a new medical practice in which patients are expected to play an active role. The paper is based on a study of the use of one specific telemonitoring device, an ambulatory ECG recorder introduced to diagnose infrequent irregularities of the heart rhythm. It seeks to examine all the invisible work it takes to produce patients who are active and responsible as participants in the diagnosis of their heart problem. In particular, I address the question of how we can understand that individuals who are anxious about their heart function manage to adopt the role of 'diagnostic agent'. This research shows that, although many patients managed to become competent users of the new technology, there are important patterns of selective use patients invented to integrate the technology in their daily life. In conclusion, the paper suggests that most patients were able to adopt the role of diagnostic agent not only because of their individual motivation but because of their location in the socio-technical network of this technology, in which the invisible work of home-care nurses and physicians at the telemedical centre made all the difference.