Atypical adenomatous hyperplasia (AAH) is believed to be the preinvasive lesion for at least a subset of lung adenocarcinomas. It was previously known as bronchial adenoma or atypical alveolar hyperplasia - terms that are no longer used. AAH is found incidentally in resected specimens. It is usually not detected by imaging techniques. In rare cases, AAH may appear as focal ground-glass opacity on a chest CT. AAH has been found in 2-4% of lungs without cancer in autopsy studies. It is more frequently found in the non-neoplastic lung in the vicinity of resected lung adenocarcinomas (up to 30% in females and 20% in males). This image is higher magnification of the previous slide and shows a focus of AAH in the center.