In recent years, assistive technologies have gained acceptance as tools for supporting chronically ill patients in achieving improvements in physical activity. However, various healthcare and sociological studies show contradicting results regarding the physical and social impact of using such devices. This paper explores real‐time user appropriation of an assistive monitoring/tracking device, the pedometer, in a healthcare intervention, with a particular focus on the technology identities users attribute to the pedometer. The study site was a rehabilitation programme at a local Danish health centre supporting patients with chronic obstructive pulmonary disease. As part of this empirical study, six focus‐group interviews were conducted with patients before and after they used pedometers. The analysis of respondents’ accounts shows that monitoring devices become part of users’ complex socio‐technical ensembles in which the use of the device and its tracking of activity is constantly negotiated through experimentation with type and frequency of use; interpretation of knowledge and experience gained via the device; and negotiation of expectations, wellbeing, and the value of quantified knowledge for the management of chronic illness. On the basis of these findings the paper brings together and advances sociological scholarship on chronic illness, embodiment, the quantified self and technology adoption.