Investing in pro-poor health services is central to poverty reduction and achievement of the Millennium Development Goals. As health care financing mechanisms have an important influence over access and treatment costs they are central to the debates over health systems and their impact on poverty. This paper examines people's utilisation of health care services and illness cost burdens in a setting of free public provision, Sri Lanka. It assesses whether and how free health care protected poor and vulnerable households from illness costs and illness-induced impoverishment, using data from a cross-sectional survey (423 households) and longitudinal case study household research (16 households). The findings inform policy debates about how to improve protection levels, including the contribution of free health care services to poverty reduction. Assessment of policy options that can improve health system performance must start from a better understanding of the demand-side influences over performance.