Background Variation in care is often poorly understood but has a big impact on patients. Non-ST segment elevation myocardial infarction (NSTEMI, also known as non-ST elevation acute coronary syndrome or NSTE-ACS) is the most common form of heart attack. NSTEMI is frequently hard to diagnose, its management pathway poorly defined and there is considerable variation in clinical practice. Methods A qualitative study based on site visits, observation, and interviews with managers, clinicians and patients. The setting was 10 hospitals in England and Wales selected to represent variation in 30-day mortality. 199 hospital staff and 68 patients were observed; 142 staff and 53 patients were interviewed. Analysis was thematic and guided by the principles of grounded theory. We triangulated interviews, observational data and medical records and interpreted these findings with reference to national guidelines. Results While the majority of hospitals in our sample had specialist cardiac roles, variation in their remits, specifically their involvement in close monitoring, significantly affected patient management. Close monitoring was important in the identification and prioritisation of patients. Rapid responses with diagnostic and treatment procedures were facilitated by close monitoring but also heavily dependent on effective and flexible bed and catheter laboratory management. Conclusions Close monitoring was a key area of variation. Guidelines for NSTEMI care specify what to do, but not how to do it. These findings are especially relevant for acute conditions with diagnostic and treatment uncertainty. Detailed examples of variation in care can inform quality improvement and potentially help improve patient outcomes.