Pulmonary adenocarcinoma-in-situ (AIS) is usually non-mucinous type consisting of mildly atypical type II pneumocytes or Clara cells lining the preexisting alveolar structures in a purely lepidic pattern. Rare cases of mucinous AIS do occur as shown here. The tumor is composed of well-differentiated tall columnar cells with basally-located small hyperchromatic nuclei. The cells have abundant intracellular mucin and may resemble goblet cells. Nuclear atypia is low-grade. Mucinous AIS must be distinguished from metastases from organs such as pancreas.