Twelve newborn infants of poorly-controlled diabetic mothers were transferred from outlying maternity hospitals for neonatal intensive care. Respiratory distress and cardiomegaly were the presenting signs. Ten infants were large for dates (macrosomic) and had echocardiographic evidence of myocardial hypertrophy, reduced ejection time, and systolic anterior movement of the mitral valve (in 6 infants). These findings are similar to those of adults with idiopathic hypertrophic subaortic stenosis. Two infants died. In survivors the myocardial hypertrophy persisted for at least 2 weeks but the evidence of functional subaortic stenosis had disappeared by 4-7 days. The 2 infants who were appropriately grown had cardiomegaly as a result of ventricular dilatation. This was associated with hypoglycaemia and acidosis, and disappeared when these metabolic disturbances were corrected.