Pulmonary PET imaging is still immature relative to developments and experience in other fields, such as brain, heart, and cancer. The reasons for this are several, including competition for access to scanner time, and a general unfamiliarity with the technology by the academic pulmonary community. These problems are likely to persist until PET is used more often in clinical or clinical research problem-solving related to lung disease. Nevertheless, the wide variety of pulmonary transport and metabolic processes amenable to study with PET should lead to continued interest and advances in the field. For instance, the advent of PET imaging to evaluate gene expression may be an especially fruitful development, given the difficulties encountered in evaluating gene therapies for such diseases as cystic fibrosis. Since the principal function of the lung is gas exchange, it is not surprising that most of the initial