While interest in social disparities in health within affluent nations has been growing, discussion of equity in health with regard to low- and middle-income countries has generally focused on north-south and between-country differences, rather than on gaps between social groups within the countries where most of the world's population lives. This paper aims to articulate a rationale for focusing on within- as well as between-country health disparities in nations of all per capita income levels, and to suggest relevant reference material, particularly for developing country researchers. Routine health information can obscure large inter-group disparities within a country. While appropriately disaggregated routine information is lacking, evidence from special studies reveals significant and in many cases widening disparities in health among more and less privileged social groups within low- and middle- as well as high-income countries: avoidable disparities are observed not only across socioeconomic groups but also by gender, ethnicity, and other markers of underlying social disadvantage. Globally, economic inequalities are widening and, where relevant information is available, generally accompanied by widening or stagnant health inequalities. Related global economic trends, including pressures to cut social spending and compete in global markets, are making it especially difficult for lower-income countries to implement and sustain equitable policies. For all of these reasons, explicit concerns about equity in health and its determinants need to be placed higher on the policy and research agendas of both international and national organizations in low-, middle-, and high-income countries. International agencies can strengthen or undermine national efforts to achieve greater equity. The Primary Health Care strategy is at least as relevant today as it was two decades ago: but equity needs to move from being largely implicit to becoming an explicit component of the strategy, and progress toward greater equity must be carefully monitored in countries of all per capita income levels. Particularly in the context of an increasingly globalized world, improvements in health for privileged groups should suggest what could, with political will, be possible for all.