Abnormal pulmonary development characterized by decreased alveolar complexity has been previously reported in patients with Down syndrome. We investigated the state of pulmonary development and found several undescribed patterns of disturbed lung growth. The axial branches of intrasegmental airways were counted in 13 Down syndrome patients; in nine, airway generation was reduced by 25% or more of the predicted number. The radial alveolar counts were evaluated in 11 lungs: five were 143% to 162% above expected (four to above adult values), five were as expected, and one was below expected (82%). No correlation was found between airway number and radial alveolar count. Our finding of the reduction in airway branching in the lungs of patients with Down syndrome suggests interference with development before birth. Disturbances in alveolar multiplication are also found.