The alveolar adenoma of the lung is a rare benign tumor in which the normal parenchymal architecture is imitated by a proliferation of both the alveolar epithelial cells and the mesenchymal septal cells. The first description, based on six cases, was published in 1986 by Yousem and Hochholzer. From their ultrastructural findings they presumed a type II pneumocytes differentiation of the epithelial cells. We investigated an alveolar adenoma of the lung immunohistochemical by means of antibodies against apoprotein B and C of human surfactant. Both the lining cells and the macrophages in the alveolar-like spaces were stained. The septal connection tissue cells did not react. These findings confirm the expression of surfactant constituents and, hence, the differentiation into type II pneumocytes of the epithelial cells of the alveolar adenoma.