In an unselected population of 2030 newborn infants studied by electrocardiography (ECG) between April 1975 and April 1977, 35 were found to have arrhythmias or other cardiac conduction abnormalities. Further investigation by means of 24-hour ECG monitoring showed that apparently serious tachyarrhythmias, such as ventricular tachycardia and slow heart rates associated with sinoatrial block, may be present without clinical disturbance in some newborn babies. Six infants had both bradycardia and tachycardia in the 24-hour recording, although the screening ECG had shown only one of these abnormalities. The alarming ECG appearance of some of the arrhythmias suggested a possible aetiological link with some unexplained sudden infant deaths: a multicentre study could determine this more readily and is therefore recommended.