English: Atypical adenomatous hyperplasia Atypical adenomatous hjyperplasia (AAH) is defined in the WHO Histological Typing of Lung and Pleural Tumors 3rd Ed as follows,: "A focal lesion, often 5 mm or less in diameter, in which the involved alveoli and respiratory bronchioles are lined by monotonous, slightly atypical cuboidal to low columnar epithelial cells with dense nuclear chromatin, inconspicuous nucleoli and scant cytoplasm. AAH is considered to be precursor of pulmonary adenocarcinoma. It is usually an incidental finding in tumor resection specimens and is separate from the adenocarcinoma. However, AAH changes may sometimes be encountered at the periphery of adenocarcinomas. In this case AAH is seen at the periphery of an adenocarcinoma. Gaps between the cells lining the alveoli are a characteristic feature of AAH..