Numerous data of the literature suggest that the cardiomegaly of hypoglycaemic newborns is due to low blood glucose levels. The size of the heart is usually determined by measuring the cardio-thoracic ratio (CTR). On the basis of the present retrospective study analyzing the clinical course and radiological findings of 66 newborns the authors suggest that: 1. CTR cannot reliably be used for determining the size of the heart in hypoglycaemic small for gestational age (SGA) newborns. 2. Routine look at the X-ray picture is also insufficient for the determination of the size of the heart in hypoglycaemic SGA newborns. 3. The method suggested by the authors is to measure the transverse diameter of the heart and to compare it to the normal, birthweight-related standard. In 3/4 of the hypoglycaemic SGA newborns studied thoracic transverse diameter (TTD) values were found to be smaller than the 50th percentile of the standard. Transverse diameter values of the heart, however, did not differ significantly from the normal mean values. In cases of true cardiomegaly, i.e. cardiac diameter above the 90th percentile, data indicative of asphyxia were more frequently seen.