Atypical adenomatous hyperplasia of the lung is a precancerous lesion of lung adenocarcinoma, but the tissue origin and specific cell type of most adenocarcinomas remain unknown, and the size is often less than 0.5 cm. Atypical adenomatous hyperplasia usually occurs in the periphery of the lung lobes, so it is difficult to detect clinically, but is often found inadvertently in lung resection specimens or on CT scans, and the images are characterized by gross sand-like changes. Microscopic histology shows intact alveolar structure with alveolar epithelial hyperplasia, which is uniformly rectangular or short columnar in shape with mild atypia, lack of or blurred nuclei, occasional focal papillary pattern in the alveolar wall, and lesions associated with pulmonary adenocarcinoma with a fine bronchoalveolar component, a presumption supported by increasing cellular anisotropy, flow cytometry, and molecular biology alterations, among other studies.