Immunosuppression occurs as a result of neutropenia (neutrophil count < 500/mm3), cellular immunodeficiency (CD4 lymphocyte count < 200/mm3) and humoral immunodeficiency (hypogamma- or dysglobulinemia), Immunocompromised patients are susceptible to infection and the lung is one of the most commonly affected organs. Bacterial pathogens are seen in the three types of immunodeficiency. Prolonged neutropenia of more than 7 days is associated with fungal infection and particularly with invasive aspergillosis. Cellular immunodeficiency is associated more frequently with tuberculosis and Pneumocysti jiroveci pneumonia. Diagnosis approach begins with looking for historical clues (immunodeficiency type, exposure...). Computed tomography features are often useful with some suggestive abnormalities (i.e. halo sign and invasive aspergillosis). Bronchoalveolar lavage is the key to microbiologic diagnosis. Hospitalization is required in most cases. Antimicrobial therapy must be introduced quickly because delayed therapy is responsible for increased mortality. Empirical treatment is used initially until specific microbiological data are available.