The indicators of social class at the individual, household, small geographical area, and at regional and national level are reviewed. The outcomes used in studying socio-economic inequalities in health and explanations of why these inequalities are generated and maintained, are discussed. Mortality in children under one year of age in relation to level ofsocio-economic development in European countries and in republics and autonomous provinces of what used to be Yugoslavia is analyzed using data frompublished sources. Gross national products per capita and national income distributions were used as indicators of socio-economic developmentin the comparison between European countries, while gross national income per capita index, the proportion of the adult population that is illiterate and proportion of the adult female population that is illiterate were used for the analysis within the ex-Yugoslavia. Finally, the trends in infant mortality in Slovenia, one of the republics of ex-Yugoslavia, are compared to trends in neighbouring countries Italy and Austria, and thesituation in regions within Slovenia is examined. Infant mortality rate in Yugoslavia has been one of the highest rates in Europe. There have been large differences in infant mortality among the republics and provinces within Yugoslavia. They may correspond to underlying differences in the level of socio-economic development.