Structural interventions addressing macro-social determinants of risk have been suggested as potentially important adjuncts to biomedical and behavioural interventions for the prevention of HIV and other diseases. A few interventions of this type have been evaluated using randomised controlled trials (RCTs), the most rigorous design to evaluate the effects of biomedical and behavioural interventions. The appropriateness of applying RCTs to structural interventions is however debated. This paper considers whether issues of ethics, feasibility and utility preclude the use of RCTs in evaluations of structural interventions for HIV prevention. We conclude there is nothing particular to this category of interventions prohibiting use of RCTs. However, we suggest that RCTs may prove unacceptable, unfeasible or not useful in certain circumstances, such as where an intervention brings important benefits other than HIV prevention (such as increased income); where leaders of clusters do not allow decisions about macro-social policies to be determined randomly; where the unit of social organization addressed by an intervention is so large that recruitment of adequate numbers of clusters is impossible; and where the period required to trial interventions extends beyond practical decision-making time-scales. In such cases, alternative evaluative designs must be assessed for their ability to provide evidence of intervention effectiveness.