Immunocompromised patients are susceptible to infections by organisms that infect individuals with normal immunity and by organisms that affect only those with abnormalities in their immune system. Using the radiographic findings and incorporating clinical information allows for the creation of a useful list differential diagnoses. The nature of the immune defect must be defined because defects in the different parts of the immune system are associated with infections by specific organisms. The time interval since transplantation provides a diagnostic clue because infections tend to occur at certain time intervals after transplantation. An epidemiologic history provides a history of exposure to organisms that may produce pneumonia. Consideration of the therapy the patient has received provides additional clues to the cause of the pneumonia. The physical examination and laboratory studies may provide an indication of the cause of the infection. Organisms tend to produce infections that have a typical rate of clinical development. Classifying the presentation as acute, subacute, or chronic provides additional etiologic clues. Correlating the clinical information with the radiographic findings generates a list of the most likely causative organisms. The list is useful in deciding on additional diagnostic tests and guiding therapy.