Specimens from 25 consecutive patients undergoing lung resection for peripheral carcinoid tumor were examined for evidence of neuroendocrine cell hyperplasia and associated obliterative bronchiolitis. Where available, the CT scans (n = 11) were reviewed for evidence of multiple tumors, and pulmonary function data (n = 16) were reviewed for evidence of airflow obstruction. Nineteen of the 25 patients (76%) had neuroendocrine cell hyperplasia in addition to the dominant carcinoid tumor. Eight patients (32%) had lesions of obliterative bronchiolitis associated with foci of neuroendocrine cell hyperplasia, and two of these patients had asymptomatic airflow obstruction that could not be related to smoking or other lung disease. We conclude that multicentric neuroendocrine cell proliferation is common in patients with peripheral carcinoid tumor of the lung. Associated bronchiolar fibrosis occurs in a high proportion of such patients, but it is usually asymptomatic.