Some patients have to work hard to manage their illness. When this work outweighs capacity (the resources available to patients to undertake the illness workload and other workloads such as that of daily life), this may result in treatment burden, associated with poor health outcomes for patients. This cross-sectional, comparative qualitative analysis uses an abductive approach to identify, characterise and explain treatment burden in chronic obstructive pulmonary disease (COPD) and lung cancer. It uses complementary qualitative methods (semi-structured interviews with patients receiving specialist care n = 19, specialist clinicians n = 5; non-participant observation of specialist outpatient consultations in two English hospitals [11 h, 52 min] n = 41). The findings underline the importance of the diagnostic process in relation to treatment burden; whether diagnosis is experienced as a biographically disruptive shock (as with lung cancer) or is insidiously biographically erosive (as with COPD).