The radiographic characteristics of 187 pristine cases of pulmonary Mycobacterium kansasii infection are reviewed. The cases were selected from a total of 309 patients with two positive sputum cultures. All but three of the 187 patients had pulmonary disease. The progenitive focus of disease almost always involved the upper lobes, the right side (72%) more often than the left (50%). Upper lobe disease began posterior to the trachea in every case. Only one patient had disease that originated in an atypical location, that is, in the superior segment of the right lower lobe. The most frequent radiographic abnormality was cavitation (96% of diseased patients). The classical description of a thin walled cavity (or cavities) occurred in 33% of patients. A cicatricial reaction was common (two-thirds of cases). About 63% of patients had endobronchial spread. The likelihood of endobronchial disease decreased as the distance from the progenitive focus increased. Pleural effusions and adenopathy were rare. In an individual case, it is not possible to differentiate between pulmonary M. tuberculosis and M. kansasii infections.