Adenocarcinoma is widely accepted to occur as a solitary, peripheral, subpleural, pulmonary mass with infrequent hilar and rare mediastinal involvement. The older literature indicates that hilar involvement occurs in 18% of cases and that mediastinal involvement occurs in only 2% of cases. A retrospective study of 100 randomly selected cases of primary adenocarcinoma of the lung diagnosed between 1976 and 1982 indicates that hilar masses are present in 40% of cases and that mediastinal masses are present in 27% of cases. In all, hilar and mediastinal masses occurred alone or in combination so that 51% of the patients in this series had plain radiographic evidence of such involvement. Since the patients in this series were studied by conventional radiographic techniques in a manner similar to patients in previous reports, it appears that adenocarcinoma of the lung may present with a much more pleomorphic radiographic appearance than was previously recognized. The reasons for this change in the radiographic appearance of adenocarcinoma are not completely understood but include a change in the histopathologic criteria for the diagnosis of adenocarcinoma and an increase in the incidence of adenocarcinoma, particularly of the poorly differentiated form.