Bronchiolar adenoma/ciliated muconodular papillary tumor is a lung neoplasm exhibiting various degrees of proximal and distal bronchiolar differentiation. Here, we evaluated distribution of MUC5AC and MUC5B in bronchiolar adenoma/ciliated muconodular papillary tumor for comparison with that seen in normal respiratory tract. In normal respiratory tract, MUC5AC was mainly distributed in large bronchi, while MUC5B was distributed in bronchi, bronchioles, and submucosal glands. In bronchiolar adenoma/ciliated muconodular papillary tumor, MUC5AC was primarily distributed in luminal cells of large airspaces, and MUC5B was distributed in luminal cells of small airspaces and mucinous glands, in addition to large airspaces, regardless of distal or proximal differentiation. In particular, MUC5B was distributed in non-mucinous club and ciliated cells in both the normal respiratory tract and bronchiolar adenoma/ciliated muconodular papillary tumor. These results indicate that MUC5AC and MUC5B distribution in bronchiolar adenoma/ciliated muconodular papillary tumor is similar to that seen in normal respiratory tract, suggestive of organoid differentiation simulating the normal lung. more...