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Fifty-three lung cancers with a pathologic diagnosis of bronchiolo-alveolar carcinoma were reviewed, particular attention being paid to the validity of the diagnosis and prognosis. Twenty-one cases were acceptable as primary lung tumors on the basis of complete necropsy study (nine cases), long-term survival following operation without recurrence (eight cases), and recurrent disease with unusually long survival times (four cases). Origin of these tumors from bronchiolar mucosa could not be proved. Contrariwise it is likely that an occasional tumor with characteristics of the terminal bronchiolar type may arise from the mucosa of small bronchi. The most consistent pattern believed to represent a bronchiologenic proliferation is the “fibroelastic” form; an associated elastica proliferation or condensation was found. Scar cancers of the classical type, i.e., central scar and peripherally oriented carcinoma, offer reassurance that the tumors are primary in the lung; their peripheral location implies that the mucosal type proliferation arises in a bronchiole. (A fibroelastic pattern was seen in some of the scar cancers in this series.) Contiguity of tumor and bronchiolar mucosa favors a primary origin for the tumor, but is not a conclusive finding at this time. A spectrum of bronchiologenic proliferation was noted, ranging from benign hyperplasia to infiltrating and destructive adenocarcinoma. Tumor growth along alveolar walls is interpreted as a superficial spreading phenomenon (“epiphytic” growth). Invasion, as opposed to superficial spread, is not always obvious with ordinary stains, but can be identified readily by elastic stains. Predominantly mucinous “epiphytic” adenocarcinomas are the ones most likely to be mimicked by metastatic cancer. Unfavorable histologic features are the presence of destructive-type invasion, intra-alveolar tumor cell clusters, multifocal tumor, blood vessel invasion, and lymph node metastases. There is a strong suggestion that scar cancers and tumors of the fibroelastic type are more amenable to resection as a group. The routine use of elastic stains will probably identify scar cancers of the condensation type that otherwise might be missed.