The diagnostic power of the heart volume and the cardiothoracic ratio in congenital heart disease in neonates has been compared. A consecutive series of 130 children with suspicion of heart disease examined radiologically at between 48 h and 15 days of age were followed for 14 +/- 10 months. Of these, 16 (12%) were diagnosed as having congenital heart disease. The number of false positive and negative diagnoses was less for heart volume than for cardiothoracic ratio using +2 SD as limit for pathology. Accuracy, sensitivity and specificity was 84, 75, and 85% respectively for heart volume and 73, 57, and 75% for cardiothoracic ratio. Cases that were false positive with both methods were significantly more often examined between 48 and 72 hours of age indicating that the explanation might be a somewhat late closure of the ductus arteriosus.