Pulmonary adenocarcinoma-in-situ (AIS) is a small (less than or equal to 3 cm), localized tumor with purely lepidic growth pattern. It is usually non-mucinous type and consists of type II pneumocytes or Clara cells with minimal atypia growing along preexisting alveolar surfaces (as seen in this image). Rare cases of mucinous type of AIS do occur. By definition, there is no stromal, vascular, or pleural invasion and there is no spread through air spaces. The patterns of invasive adenocarcinoma (acinar, papillary, micropapillary, solid, colloid, enteric, fetal, or invasive mucinous adenocarcinoma) are absent.