In the 28th week of gestation a normal foetus and a holoacardius were diagnosed via sonography in a biamnotic monochoriatic twin pregnancy. The growth of the foetuses was observed under continuous sonographic control up to the 40th week of pregnancy. The patient delivered a healthy twin by Caesarean section and the acardius was subjected to a postmortem. Macroscopic and histological findings in the foetus, the placenta and the umbilical cords are demonstrated. The pathogenesis of the holoacardius is discussed as being due to chromosomal aberrations and foeto-foetal transfusion syndrome.