Adenocarcinoma in situ of the lung is no longer classified as a cancer (malignancy) in the latest WHO histologic classification of lung tumors. Adenocarcinoma in situ (AIS) is defined as adenocarcinoma cells growing entirely along the alveolar wall (adherent-like) without interstitial, vascular, or pleural invasion, with no mixing of follicular, papillary, solid, or micropapillary components, no tumor cells in the alveolar lumen, and a maximal diameter of the tumor of ≤3 cm. In the latest WHO classification, adenocarcinoma in situ of the lung is classified as a glandular precursor lesion, and the prognosis of this disease is very good, and after surgical resection, the disease-free survival rate can reach 100%. However, patients with adenocarcinoma in situ should not take it lightly, and it is recommended to follow the doctor’s prescription for timely treatment to avoid developing into invasive lung adenocarcinoma.