This paper is an empirical examination of the determinants of aggregate health care expenditure. The paper presents a systematic analysis of relationships across 19 OECD countries, showing the effects of aggregate income, institutional and socio-demographic factors on health care expenditure. The results indicate that institutional factors of the health systems, in addition to per capita Gross Domestic Product (GDP), contribute significantly to the explanation of the health care expenditure variation between countries; for example the way physicians in outpatient care are paid, and the mixture of public/private funding and inpatient/outpatient care.