Atypical adenomatous hyperplasia (AAH) is believed to be the preinvasive lesion for at least a subset of lung adenocarcinomas. It is a slow-growing lesion, usually found incidentally in resected specimens. The most common location is peripheral lung, often close to a respiratory bronchiole. It shows proliferation of mild to moderately atypical type II pneumocytes (cuboidal) or Clara cells (columnar) along alveolar surfaces. The degree of cytologic atypia may vary, but grading of AAH is not recommended. If completely resected, the disease-free survival is 100%.