OBJECTIVE: Closure of the fetal ductus arteriosus, which is usually due to nonsteroidal antiinflammatory agents, may be detrimental. Therefore prenatal and postnatal clinical and echocardiographic findings in four human fetuses with spontaneous ductus arteriosus occlusion are reported. STUDY DESIGN: Echocardiographic and clinical data were retrospectively analyzed. RESULTS: Spontaneous closure of the ductus arteriosus was discovered in four fetuses (gestational age 34 to 38 weeks). No mother had received nonsteroidal antiinflammatory agents. Enlargement of the right heart and pulmonary arteries and tricuspid and pulmonary regurgitation were present in all cases. Two fetuses had right ventricular hypertension. Postnatally their right ventricular function recovered promptly. The others had severe right heart failure with abnormal umbilical venous pulsations. After immediate delivery none had signs of persistent pulmonary hypertension. However, they have echocardiographic evidence of right ventricular dysfunction 2 to 6 months after delivery. CONCLUSIONS: Occlusion of the fetal ductus arteriosus may also occur in the absence of treatment with nonsteroidal antiinflammatory agents. Immediate delivery resulted in good clinical outcome, although right ventricular dysfunction may persist. (AM J OBSTET GYNECOL 1996;174:879-83.)