This image shows a focus of diffuse idiopathic pulmonary neuroendocrine cell hyperplasia (DIPNECH) in the wall of a bronchiole. The neuroendocrine cells are no longer confined to the mucosa and appear to be infiltrating into the surrounding parenchyma forming a tumorlet. The tumorlets have irregular infiltrative margins, are associated with an airway, and are 5 mm or less in size. The etiology of DIPNECH is unknown. The lesions may be asymptomatic and discovered incidentally on high-resolution chest CT OR they may cause cough, breathlessness, and wheezing secondary to inflammatory and fibrotic changes in the airways. Clinical significance of DIPNECH: DIPNECH is a slowly progressive, chronic disease that may result in obliterative bronchiolitis. It is thought to be a precursor to a minority of carcinoid tumors. Most of these are typical carcinoids with an indolent course, but rare instances of atypical carcinoids have been documented. There are no documented cases of small cell carcinoma and large cell neuroendocrine carcinoma arising in the setting of DIPNECH.