Unilateral developmental deficiency of lung is rare. As the most of those, unilateral agenesis or Scimitar syndrome(hypogenetic lung syndrome) had been reported. We experienced that two cases of unilateral developmental deficiency of lung, which is different form each other, are found in identical twin. To our knowledge, this condition has not been reported before. We evaluated lung parenchyma and bronchus(; simple chest PA, chest CT(HRCT, 3DCT)), vasculatures(; pulmonary angiography and cardiac catheterization) and combined anomalies (;abdominal ultrasound, echocardiography, routine blood chemistry, and chromosomal study). This 31 yrs old twin female patient was confirmed as an unilateral agenesis of left upper lobe and hypoplasia of left lower lobe without combined anomalies. She's twin younger sister was confirmed as unilateral agenesis of right lung with atrial septal defect(ASD).