BACKGROUND: The actual yield from current screening for clinically significant congenital heart malformations in Dutch child health care is far from optimal. In this study factors that determine the effectiveness of this screening are identified and recommendations for the optimization of the screening programme are formulated. METHODS: Eighty-two patients with a clinically significant congenital heart malformation were consecutively included in this study. Parents and child health centre physicians were interviewed in order to establish the screening, detection and referral history. Paediatric cardiologists established whether these patients were diagnosed 'in time' or 'too late'. RESULTS: Incomplete performance of the screening examination has more influence on the occurrence of delayed diagnoses than failure by parents to adhere to the complete visit schedule. Adequate screening advances detection of congenital heart malformations. Severity, however, is the most predominant determinant of the age at referral and diagnosis, as well as of the risk of complications. In only 7 out of 39 patients diagnosed 'too late', could no avoidable cause for an adverse outcome be found. In 10 cases (25%) there was a prolonged interval between first referral and diagnosis. CONCLUSION: To optimize the yield of the screening programme, improvement in the performance of the child health centre physicians and the co-operation of other physicians involved in reducing the interval between referral and diagnosis are required. Thus a considerable improvement in the prevention of complications of congenital heart malformations can be obtained.