This qualitative study explored parental decision-making about the DTaP/IPV/Hib 'five-in-one' vaccine. Semi-structured interviews were conducted with 22 parents of babies aged between 4 and 13 weeks old, recruited from four practices in southern England. A modified Grounded Theory approach identified that although parents had some concerns, most complied with the recommended programme rather than making an informed decision. Other themes related to perceived importance of immunisation; beliefs about how immunisation works; trust; perceptions of vulnerability; feelings of guilt and responsibility; and practicalities. It is important to explore how parents' attitudes change over the preschool years and to develop ways of addressing uncertainties about immunisation, including the safety of combining antigens and the need for boosters.