This study explores Thai physicians' rationales about their prescribing practices for treating childhood diarrhoea within the public hospital system in central Thailand. Presented first are findings of a prospective clinical audit and observations of 424 cases treated by 38 physicians used to estimate the prevalence of sub-optimal prescribing practices according to Thai government and WHO treatment guidelines. Second, qualitative interview data are used to identify individual, inter-personal, socio-cultural and organisational factors influencing physicians' case management practices. Importantly, we illustrate how physicians negotiate between competing priorities, such as perceived pressure by caretakers to over-prescribe for their child and the requirement of health authorities that physicians in the public health system act as health resource gatekeepers. The rationales offered by Thai physicians for adhering or not adhering to standard treatment guidelines for childhood diarrhoea are contextualised in the light of current clinical, ethical and philosophical debates about evidence-based guidelines. We argue that differing views about clinical autonomy, definitions of optimal care and optimal efficiency, and tensions between patient-oriented and community-wide health objectives determine how standard practice guidelines for childhood diarrhoea in Thailand are implemented.