Recently Ferencz et al. [6] using the datas of the Baltimore-Washington Infant Study found a predominance of double outlet righ ventricle and truncus arteriosus communis among the congenital heart diseases of newborns of diabetic mothers. This paper reports about the prenatal diagnosis of a truncus arteriosus communis Type I in a 23 year old pregnant with insulin-dependent diabetes mellitus (White B). The diagnosis was performed in the 25th week using Real-Time- and Color-Doppler-sonography. The important features in the prenatal differentialdiagnosis from a tetralogy of Fallot are explained. The optimal management of pregnancy, the delivery at a perinatal center with subsequent operative correcture could improve the prognosis of the newborn.