BACKGROUND Bronchiolar adenoma (BA) and ciliated muconodular papillary tumor are rare tumors that have bilayered cell proliferation and continuous expression of p40 and CK5/6 in the basal cell layer. Diagnosis is difficult because of the limited knowledge of these tumors and their morphological similarities to malignant tumors, including invasive mucinous adenocarcinoma, especially based on the histopathology of intraoperative frozen sections. These tumors are now considered to be benign neoplasms, with malignant transformation reported in only a few cases. CASE SUMMARY A 57-year-old woman presented with a 17.0 mm × 7.0 mm nodule in the lower lobe of the left lung. Hematoxylin-eosin staining and immunohistochemistry of a surgical specimen were performed. The tumor consisted of a BA area and a mucinous adenocarcinoma (MA) area. In the BA area, the tumor had a bilayered structure of luminal cells and basal cells. The basal cells were positive for CK5/6 and p40, but the MA area was negative for these biomarkers. The Ki-67 proliferation index was low (1%-2%). The patient was diagnosed with BA accompanied by MA, and had a favorable outcome. CONCLUSION The present study indicated that BA may be carcinogenic, and suggests that clinicians should be aware of its potential for malignant transformation.