Collateral ventilation was determined in 13 isolated human lungs obtained at postmortem. These included lungs from patients with pneumonia (2 cases), metastatic breast carcinoma (3), emphysema (5), pulmonary fibrosis (2) and hemorrhagic abscess (1). The amount of collateral ventilation was variable and related to the pathological process within the lung. Flow and resistance of collateral pathways were determined for each studied lung. Demonstration of collateral flow was made with the use of 133Xe and recorded by radiographic imaging. Further demonstration of the collateral pathways was done by use of 1 micrometer sized particles of tantalum. Histological examination showed tantalum dust to accumulate at gaps in the alveolar wall at the lobar junction and to pass through this area. Although the sample population is small, no correlation was found for age and the amount of flow through the channels. This study demonstrated that alveolar pores play a role in collateral ventilation. No collateral flow was observed in six pediatric lungs tested. It is likely that collateral ventilation is an acquired phenomenon in humans.