Traditionally, medical professionals have entrusted the parents of children with chronic illness with the task of imparting knowledge about the illness to the children. This practice assumes that parents understand the illness, and that they pass on their knowledge in a manner appropriate for the individual child. The aim of our study was to assess the knowledge that parents of children with a cardiac defect have about the malformation in the heart of their child. We sent a modified version of the Leuven questionnaire to 350 families. The questionnaire was filled in and sent back to our centre by 148 families. Parents showed a good knowledge of the name and anatomical characteristics of the cardiac disease suffered by their child. Parents with a child taking drugs were not very knowledgeable about the regime, side effects, and interaction with other drugs or food. Only one-quarter of the parents knew the definition of endocarditis, although almost two-fifths were aware that unexplained fever for more than 5 days was the most typical symptom. Less than half of the parents knew that endocarditis could recur. About two-fifths of parents knew the real possibility of their child being involved in competitive sports; but almost half of parents were unable to answer this question. The poor knowledge about particular aspects of the disease, treatment, and preventive measures revealed by our parents may have major consequences. The results of our study are relevant to general daily clinical practice.