This paper examines the utility and actual usage of evidence from trials of social interventions. It focuses on two case studies of trials of HIV prevention funded by the UK's NHS, one in a genitourinary medicine clinic, and another in a voluntary agency. The research concludes that a variety of contextual factors meant that the trials did not produce very useful findings. The interventions chosen were not always ideal candidates for evaluation. Although process evaluation did occur this was not reported in such a way as to enable consideration of the potential generalizability of the interventions. Although neither trial was led solely by academic researchers, certain stakeholders were excluded from the design and undertaking of both trials. Although findings from both trials were used in informing (or supporting) decisions about the planning and implementation of services, this did not occur in the manner that proponents of evidence-based practice might hope for.