Four infants with the severe form of the syndrome of tetralogy of Fallot and absent pulmonary valve are described. All had severe respiratory distress due to bronchial compression from massively dilated pulmonary arteries. The clinical diagnosis was confirmed by cardiac catheterization and angiocardiography. The infants died at age 3, 5, 30 and 90 days, respectively. Autopsy revealed, in addition to the usual components of the syndrome, absence of the ductus arteriosus or its remnants. The possible relation of "agenesis" of the ductus arteriosus to the pathogenesis of the severe form of this syndrome is discussed and pertinent studies are reviewed.