Pulmonary infections are second in importance only to septicemia as a cause of infectious morbidity and mortality in patients with hematological disorders. The differential diagnosis of the pneumonitis syndrome includes not only infection but also a multitude of noninfectious causes. In addition, the diagnosis may be difficult, owing to the subtlety of the clinical signs as a consequence of the impaired inflammatory response. Radiographic findings are often nonspecific, and invasive procedures and microbiological exams are required to establish the cause of pulmonary disease and to choose a specific therapy. However, invasive diagnostic procedures are often precluded by the poor general conditions and (particularly in acute leukemia patients) by concurrent thrombocytopenia. The approach to all infectious complications, including those of the lower respiratory tract, in immunocompromised patients with hematological diseases, is based on aggressive prevention strategies and the empirical administration of broad-spectrum antimicrobials eventually followed by a clinically or microbiologically guided treatment modification. With regard to the antimicrobial treatment, given the variety of infectious and noninfectious causes of pulmonary infiltrates in patients with hematological diseases, the diversity of the underlying immunocompromised state, and the spectrum of clinical findings, no single general therapeutic algorithm can be applied.