Ciliated muconodular papillary tumors (CMPTs) are rare peripheral nodules of the lung first described in 2002. Because of their rarity and nonstandardized diagnostic terminology, CMPTs have been poorly recognized among pathologists. To better characterize these lesions, we undertook a detailed clinicopathologic and immunohistochemical study of 10 archival cases. Ten CMPTs occurred in 7 men and 3 women with a median age of 62 years. All were small peripheral nonendobronchial nodules with a mean diameter of 1.0 cm. All but 1 tumor were incidentally detected by computed tomography-based screening, all of which were radiologically interpreted as adenocarcinomas. Although limited surgery treated all but 1 CMPT, they followed a benign course with no recurrence at a mean follow-up of 43 months (range: 2 to 88 mo). Histologically, CMPTs showed glandular and/or papillary architecture, comprising a vaguely organized mixture of nonatypical ciliated columnar cells, mucous cells, and basal cells, often enveloped by copious intra-alveolar mucin. Micropapillary tufts of ciliated cells and seemingly discontinuous growth along alveolar walls were occasionally present, mimicking adenocarcinomas. Ciliated cells and basal cells were immunopositive for TTF-1 and p40, respectively, whereas mucous cells lacked HNF4α expression. CMPTs are rare, likely benign, underrecognized processes of the lung that should be distinguished from adenocarcinomas.