Absent semilunar valve was found in three fetal cases with cystic hygroma. Two cases which simultaneously showed absent aortic and pulmonary valves (AAV and APV, respectively) had double-outlet right ventricle. The third case, which lacked only the aortic valve, had atrioventricular septal defect and anomalous origin of the right subclavian artery. Two of the three cases had a markedly hypoplastic thymus. Fifteen AAV and 179 APV cases, including the above-mentioned cases and others reported elsewhere, were discussed with special reference to the pathogenesis of absent semilunar valve. Of the 15 AAV cases, hypoplasia of the left heart was observed in 11 cases (73.3%), double-outlet right ventricle in 5 (33.3%), and aortic arch malformations in 6 (40.0%). In the 179 APV cases, there were 111 tetralogy of Fallot (62.0%) and 44 right-sided aortic arch (24.6%). DiGeorge anomaly was found in one AAV and eight APV cases. These results indicate a pathogenesis that is possibly related to hemodynamic abnormalities or abnormal neural crest cells. Further investigation will be needed to elucidate a more definite pathogenesis of absent semilunar valve.