AIM: Advances in medical treatment in recent years have led to dramatically improved survival rates of children with severe congenital heart defects (CHD). However, very little is known about the psychological consequences for these children, particularly during and after the early period of invasive treatment. In this study, we investigated the extent to which the severity of the CHD affects the child's emotional reactivity at 6 months of age. METHOD: We linked prospective data from the Norwegian Mother and Child Cohort Study, conducted by the Norwegian Institute of Public Health with a nationwide medical CHD registry and identified 212 infants with CHD in a cohort of 61 299 infants. Mothers reported on their child's emotional reactivity at age 6 months by means of a standardized questionnaire. RESULTS: Infants with severe to moderate CHD had 60% higher odds for severe emotional reactivity (cut-off at the 85 percentile) compared with healthy infants, after controlling for important maternal and child confounders. CONCLUSION: Our study is the first to show elevated emotional reactivity in children with moderate to severe CHD, suggesting a need for special parental attention to soothe their distress. Follow-up studies will show whether this emotional reactivity is transient or an early marker of continuing emotional or behavioural problems.