This report describes six cases of an unusual pulmonary neoplasm, the alveolar adenoma, that consists of a benign proliferation of alveolar epithelium and septal mesenchyme. The neoplasm presents as a solitary peripheral lesion on chest radiographs, particularly in older patients. It has unique histologic features, which should be distinguished from those of sclerosing hemangioma, lymphangioma, and bronchioloalveolar carcinoma.