This paper reflects on the conduct of a pilot study which was intended to prepare the ground for a randomized, controlled clinical trial of the assessment and treatment of patients with minor injuries by nurse practitioners or senior house officers (junior doctors) in an accident and emergency department. The authors believe that such reflection makes a useful contribution to the growing debate about methodology in health services research. Following a brief literature review on nurse practitioners, the intended research design is explained. The authors then describe how observation in the chosen department led to a change in design, involving trial use of patient-kept diaries as a means of assessing outcome. Despite promising results from the third diary exercise, doubts remained about the validity of the relationship between process and outcome. The small number of patients managed by nurse practitioners compared to the greater number managed by junior doctors posed difficulties for randomization; similarities in pathways of care for patients managed by the two staff groups made it unlikely that differences in outcome could be demonstrated. These problems led to abandonment of the plan for a clinical trial at that point, but the authors believe that further exploration of the idea is desirable.