Recent debates in the United Kingdom about the measles, mumps and rubella (MMR) vaccine and its alleged link with autism have centred on contested notions of risk. This paper presents findings from 87 parents' focus group and interview discussions of their decision-making about the vaccine in light of three streams of theoretical literature on risk (cultural theory, risk society, psychometric models of risk perception) and models of vaccination acceptance and resistance. In addition to the risks of infectious disease and autism, parents balanced other risk concerns-both biological and social-in making their decisions. Such decisions, made on behalf of children unable to choose for themselves, and in the midst of contradictory information and uncertainty, symbolised what it means to be a 'good parent'. To cope with uncertainty, parents sought explanations for why some children seem to be more vulnerable to adverse outcomes than others. Debates about children's risks may need special theoretical consideration beyond that offered by the current risk literature. Specific aspects of the MMR debate, namely, selecting between potentially competing risks, making risk judgements on behalf of dependent others, and tensions between private and public good, provide a platform for exploring how social theories of risk might be adapted for children's health controversies.