Adenocarcinoma in situ (AIS) of the lung —previously included in the category of "bronchioloalveolar carcinoma" (BAC)—is a subtype of lung adenocarcinoma. It tends to arise in the distal bronchioles or alveoli and is defined by a non-invasive growth pattern. This small solitary tumor exhibits pure alveolar distribution (lepidic growth) and lacks any invasion of the surrounding normal lung. If completely removed by surgery, the prognosis is excellent with up to 100% 5-year survival.

Although the entity of AIS was formally defined in 2011, it represents a noninvasive form of pulmonary adenocarcinoma which has been recognized for some time. AIS is not considered to be an invasive tumor by pathologists, but as one form of carcinoma in situ (CIS). Like other forms of CIS, AIS may progress and become overtly invasive, exhibiting malignant, often lethal, behavior. Major surgery, either a lobectomy or a pneumonectomy, is usually required for treatment.