Delayed detection of coarctation in infancy: implications for timing of newborn follow-up. During a recent 5-year period, 74 patients younger than 6 months of age were diagnosed with coarctation of the aorta. Coarctation was correctly diagnosed in only 22% of patients prior to referral despite readily apparent femoral pulse abnormalities in 86%. Infants whose symptoms were detected between 5 and 14 days of age were significantly more ill than infants outside this age range and had a high mortality rate (25%). The number of associated cardiac defects was not related to the severity of clinical illness in this group, suggesting that closure of the ductus arteriosus is the primary determinate of disease severity. Observations in two patients suggested that a detectable pulse discrepancy occurs between 3 and 5 days postnatally. Upper extremity hypertension was found commonly in infants after 5 days of age despite the presence of congestive heart failure. Earlier detection of coarctation in the newborn requires a diligent cardiovascular and peripheral pulse examination between 3 and 7 days of life, upper extremity and lower extremity blood pressure measurement, and a high index of suspicion. 