The halo sign is a circular area of ground-glass attenuation that is seen around pulmonary nodules at computed tomography (CT). Although the sign is most often an indication of pulmonary hemorrhage, it may also accompany other lesions associated with different disease processes. Examples are hemorrhagic nodules of infectious origin (mucormycosis, candidiasis, tuberculosis, viral pneumonia, and invasive aspergillosis--the last being the most common cause of the CT halo sign); hemorrhagic nodules of noninfectious origin (Wegener granulomatosis, Kaposi sarcoma, and hemorrhagic metastases); tumor cell infiltration (bronchioloalveolar carcinoma, lymphoma, and metastasis with intra-alveolar tumor growth); and nonhemorrhagic lesions (sarcoidosis and organizing pneumonia). Diagnosis must therefore be based on careful consideration of all the CT chest findings within the context of the patient's clinical state. The aim of this review was to describe and illustrate different disease processes that appear as a halo sign on CT scans, to analyze the value of this diagnostic tool, and to assess its correlation with pathology findings.