OBJECTIVE: To assess how much adults with congenital heart disease understand about their heart defect, its treatment, and the preventive measures necessary to avoid complications. DESIGN: Descriptive, cross sectional study. SETTING: Adult congenital heart disease programme in one tertiary care centre in Belgium. PATIENTS: 62 adults with congenital heart disease (47 men; 15 women), median age 23 years. MAIN OUTCOME MEASURES: Patients' knowledge was assessed during an outpatient visit using the Leuven knowledge questionnaire for congenital heart diseases, a 33 item instrument developed for this study. RESULTS: Patients had adequate knowledge (> 80% correct answers) about their treatment, frequency of follow up, dental practices, occupational choices, appropriateness of oral contraceptives, and the risks of pregnancy. Knowledge about the name and anatomy of the heart defect, the possibility of recurrent episodes of endocarditis during their lifetime, and the appropriateness of different physical activities was moderate (50-80% correct answers). There was poor understanding (< 50% correct answers) about the reasons for follow up, the symptoms of deterioration of the heart disease, the definition, characteristics, and risk factors of endocarditis, the impact of smoking and alcohol on the heart disease, the hereditary nature of the condition, and the suitability of intrauterine devices as contraceptives. CONCLUSIONS: Adults with congenital heart disease have important gaps in their knowledge about their condition. The results of this study can be used as a basis for developing or optimising structured educational interventions to enhance patients' health behaviour.