Invasive pulmonary aspergillosis (IPA) in immunocompromised patients is often difficult to diagnose. Many pathogens present initially with similar, nonspecific pulmonary findings. Air crescent formation has been reported to be highly suggestive of IPA in the appropriate clinical setting, but this is a late sign in an otherwise rapidly fatal infection. The authors reviewed the available chest computed tomography (CT) scans of nine patients with acute leukemia and documented IPA, including four patients with serial scans obtained during the course of infection. Typical CT findings of IPA were multiple inflammatory nodules, often with one large dominant mass, or a single peripheral masslike infiltrate. Cavitation or air crescent formation occurred late in the course of infection, usually at the time of bone marrow recovery from chemotherapy. CT scans obtained early in the course of infection in two patients demonstrated a distinctive feature of one or more pulmonary masslike infiltrates surrounded by a halo of low attenuation. These lesions subsequently progressed to cavitation or air crescent formation typical of IPA. While this CT halo sign may not be pathognomonic for Aspergillus, seen in the appropriate host, it may suggest early on the possibility of IPA.