Two-year old monozygotic female twins were admitted to the Nihon University Hospital because of recurrent respiratory infections. The chest X-rays showed a similar pattern in both twins. The left lung fields were almost totally opaque and the right lung fields were emphysematous. Pulmonary inhalation 81 mKr scintigrams revealed limited ventilation in left lower lungs. Cardiac catheterization demonstrated no intra-cardiac shunting and normal pulmonary artery pressures in both twins. Pulmonary arteriograms showed mildly dilated right pulmonary arteries and hypoplastic left pulmonary arteries which distributed to left lower lungs. The angiograms gave also a quite similar pattern in both twins.Plenty of hypoplasia of a lung have been reported already. However, the reports on the twins are quite rare. To our best knowledge only 9 cases have been reported, and in most of the reports only two twins are involved. This is the first case of hypoplasia of the lungs in twins in which both twins were similarly involved in Japan.A prognosis of this kind patient is obscure. (Acta Paediatr Jpn. 23(3): 404, 1982) (Abstract of Nippon Shonikagakkai Zasshi (Acta Paed. Jpn Japanese ed. 85(11): 1560-1565 1981)) Acta Paediatr Jpn