The parents or guardians of attenders at a paediatric cardiac clinic were surveyed over five months. Ninety-six patients who were at risk of bacterial endocarditis were identified. In 46% of these, the parents or guardians had insufficient knowledge to ensure that their children will receive prophylaxis at times of risk. A first language other than English, a lower level of education of the main family wage- earner , lack of exposure to previous dental extractions and non-cardiac operations, and a child aged less than five years--all correlated with poor knowledge of prophylaxis. Means of achieving greater awareness of need and efficacy of protection are suggested.