Pulmonary infection in immunocompromised patients is frequently difficult to diagnose. Therapy for the more common pathogens differs greatly from that for infection with unusual opportunistic organisms. However, neither of these infectious agents offers specific radiographic signs. The authors report on 4 patients with acute leukemia and invasive aspergillosis whose radiographs demonstrated a distinctive feature of one or more air crescents within an area of pulmonary infiltrate. Autopsy studies correlated the radiographic changes with an infection due to Aspergillus species fungi. While the sign is not pathognomonic for Aspergillus infection, seen in a suitable host, it would suggest the possibility of invasive aspergillosis.