In this article the authors argue that the social value of health research should be conceptualized as a function of both the expected benefits of the research and the priority that the beneficiaries deserve. People deserve greater priority the worse off they are. This conception of social value can be applied for at least two important purposes: (1) in health research priority setting when research funders, policy-makers, or researchers decide between alternative research projects; and (2) in evaluating the ethics of proposed research proposals when research ethics committees (RECs) assess whether the social value of the research is sufficient to justify the risks and burdens to research participants and others. In assessing how far a proposed research project will advance the interests of people who are more disadvantaged, research priority setters and RECs should examine (at least) the diseases that the research targets and the type of research. Just as certain diseases impose a greater burden on people who are more disadvantaged, so certain types of intervention and forms of research are more likely to benefit people who are more disadvantaged. The team outline which populations are likely to be representative of the global worst off and identify what types of health research, and which disease categories, are priorities for these populations.