Alveolar adenoma of the lung is a rare benign neoplasm with proliferation of alveolar epithelium and septal mesenchyme. This unusual neoplasm presents as a solitary mass on chest roentgenograms, especially in older, asymptomatic patients. We report a case of this unusual neoplasm and describe the histologic and radiographic features of this tumor, which should be included in the differential diagnosis of a solitary pulmonary mass.