In recent years there has been an increasing incidence of Pseudomonas aeruginosa pneumonia. The factors responsible for this include the present-day common usage of antibiotics, steroids, cytotoxic and immunosuppressive drugs. Of particular importance, however, is the widespread use of positive pressure breathing apparatus. Pulmonary involvement may occur as part of a generalized Pseudomonas aeruginosa septicemia or as a primary nonbacteremic pneumonia. The increasing incidence and gravity of established Pseudomonas aeruginosa pneumonia are providing a major problem in many intensive care units. The present report is essentially concerned with a consideration of the various roentgenologic changes which may be observed in primary Pseudomonas aeruginosa pneumonia, and is based upon the findings in a series of patients who developed this complication while receiving mechanical ventilation in an intensive care unit. The roentgenologic differential diagnosis is briefly discussed.