A systematic analysis was made of the autopsies of 74 newborns and fetuses (49 pathological cases and 25 controls) to detect defects in pulmonary growth. Radial alveolar count (RAC) and histological study were made in each case, as well as measurement of the ratio of pulmonary weight to total body weight (R). The ratio is of interest when lower than 0.012, but has no value if there is an intercurrent pathology. RAC decreases in pulmonary hypoplasia, but is not an entirely reliable indication since there is evolution of the values as pregnancy advances and a greater dispersion of results the earlier the gestational age. Histology disclosed an excess of bronchi, including distal bronchi, some of which exhibited delay in the differentiation of the distal aerian channel. The analysis of any two of the three above will enable diagnosis of pulmonary hypoplasia.