Unilateral lung agenesis should be suspected in fetuses presenting mediastinal shifting and absence of evidence of diaphragmatic hernia. Confirmation of diagnosis is not possible until birth as other more common lung anomalies may have a similar appearance. We present a case with right lung agenesis diagnosed at 34 weeks of gestation. Important for precise diagnosis was the color Doppler procedure, which revealed the absence of right pulmonary vessels. This is the first case in which unilateral lung agenesis has been recognized prenatally by nonvisualization of the corresponding lung vessels. The prenatally performed magnetic resonance sonography did not offer any advantages over sonography. Postnatally, the diagnosis was confirmed, and sinus venosus defect was also found. To prevent recurrent cardiopulmonary crisis, which could be caused by mediastinal shift, a tissue expander was implanted successfully at 3 months of age.