Pulmonary endocrine cells have been studied according to their immunoreactivity for neuron-specific enolase, gastrin-releasing peptide (GRP), calcitonin, leucine-enkephalin, and serotonin in the lungs of subjects with chronic bronchitis and emphysema. The overall population was significantly greater in comparison with matched controls. The change was most marked in lobes affected by pneumonic consolidation, and did not affect equally the sub-populations of cells as identified by their content of peptide; the difference was much greater for calcitonin-containing cells than for those immunoreactive for GRP. This change may be the basis of the hypercalcitoninaemia and hypercalcitoninuria which have been reported in patients with inflammatory pulmonary disease.