The recent controversy surrounding the safety of the measles, mumps, and rubella vaccine (MMR) has heightened parents' concerns about the safety of vaccines, and led some to believe that giving vaccines in a combined form may 'overload' children's immune systems. However, to date no studies have been published examining how British parents conceptualise the notion of 'immune-overload' or how they relate this concept to their own children. Eighteen focus groups were conducted with parents between November 2002 and March 2003. The literature on vaccine decision-making suggests that parents base their immunisation decisions on two key risks: those posed by the diseases, and those associated with the vaccines aimed at preventing those diseases. Our study suggested that for some parents a third factor plays an important role, namely their assessment of the ability of their child's immune system to 'cope' with the challenge of combined vaccines, or to fight the disease. We conclude that although there is no scientific evidence that supports parents' fears about combined vaccines causing 'immune-overload', policy makers need to recognise these concerns if they are to successfully persuade parents that combined vaccines are safe.