OBJECTIVE: To explore parents' understandings of the diseases included in the current UK Childhood Immunization Programme (CIP), and the role of first- and second-hand experiences of these diseases in assessments of their severity. METHODS: A qualitative study in which 66 parents (58 mothers and 8 fathers) of children aged 6 years and below, and six mothers of immuno-compromised children, took part in 18 focus group discussions between November 2002 and March 2003. RESULTS: There were many gaps in parents' knowledge about some vaccine-preventable diseases, most notably diphtheria, tetanus and haemophilus influenzae type b, three of the diseases covered by the pentavalent vaccine (introduced into the UK CIP in 2004). These gaps led some parents to question the need for vaccination. First-hand experiences of the diseases reinforced the need for vaccination in some cases (e.g. Men C), but undermined it in others (e.g. pertussis, measles, rubella, mumps). Poliomyelitis and diphtheria were no longer seen as a threat to children's health in Britain. Some parents saw mumps as only a threat to boys' health and rubella as only having relevance to girls'. CONCLUSIONS: As fewer parents have direct experiences of vaccine-preventable diseases, there is an increasing need to provide parents with accessible information about these diseases. It is also important to recognize that direct or indirect experiences of any of the diseases may either heighten or diminish parents' assessments of the severity of these diseases.