Alveolar adenoma is a rare and benign tumour of the lung that usually presents in asymptomatic patients as a coin lesion on chest radiography. Only 25 cases have been reported in the English medical literature. Alveolar adenoma has a characteristic multicystic histology and often resembles the normal lung parenchyma. Ultrastructural studies indicate that the epithelial cells lining the cysts are type-II pneumocytes. Immunohistochemical analysis may aid in the characterization of alveolar adenoma and discriminate this condition from other types of benign lesions of the lung. An indolent clinical progression and absence of recurrence and metastasis after complete resection are the most important characteristics indicative of the benign nature of alveolar adenoma. Few studies have been conducted at the molecular level, such as by flow cytometry, with the objective of characterizing the biological nature of alveolar adenoma. Differential diagnoses include sclerosing hemangioma, papillary adenoma, lymphangioma, atypical adenomatous hyperplasia and bronchioloalveolar carcinoma. In this article we describe the immunohistochemical and flow cytometric features of this neoplasm in two male patients. Both the tumours showed a diploid DNA pattern with a low proliferation index. p53 test was found to be negative, and post-operative follow-up examinations at 22 and 32 months proved uneventful.