The higher occurrence of structural heart disease in fetuses of previous diabetic mothers is well described in the literature, This study reviews the fetal echocardiograms of 1075 diabetic mothers (852 gestational diabetes and 223 previous diabetes) examined at the Fetal Cardiology Unit from January 1989 - October 1996. There were 162 cardiac abnormalities (15%), excluding rhythm disturbances and functional disorders. The abnormalities included in the study were 120 hypertrophic cardiomyopathy and 35 structural heart diseases. In the gestational diabetes group (n = 852) there were 87 (10.2%) patients with hypertrophic cardiomyopathy and 25 (2.9%) cases of fetal cardiac malformations: ventricular septal defects-VSD (18), double outlet right ventricle-DORV (3), pulmonary atresia-PA (1), double inlet left ventricle-DILV (1), complete atrioventricular septal defect-CAVSD (2). In the previous diabetes group (n = 223) there were 33 (14.7%) fetuses with hypertrophic cardiomyopathy and 17 (7.6%) cases of structural heart disease: VSD (14), tetralogy of Fallot-TF (1), DORV (1), pulmonary stenosis (1), truncus (1), mitral atresia (1), transposition of the great arteries - TGA (1), dysplastic tricuspid valve (1). Even though the prevalence of hypertrophic cardiomyopathy in both groups was predictable, there was a much higher than expected prevalence of fetal structural heart disease in the group of mothers with gestational diabetes. This finding may raise the suspicion of an erroneous diagnosis of gestational diabetes or suggest that some teratogenic effect would already be present early in gestation.