The authors describe the computed tomographic (CT) appearances of aspergilloma in 26 patients. With narrow (3-mm) sections, a bone algorithm, and wide window settings, CT scans enabled accurate identification of both the forming aspergilloma as well as the mature fungus ball in 25 cases. The diagnostic accuracy of CT was confirmed histologically in nine cases. The radiologic concept of the aspergilloma as a solid mass partially surrounded by a crescent of air is no longer tenable as the only definite criterion for diagnosis. In cases in which this classic appearance is seen on CT scans, mobility is easily demonstrated with use of prone and supine positions. In other cases, the aspergilloma appears as an irregular spongework containing air spaces and filling the cavity, obliterating the air crescent sign. The fungus ball is therefore fixed and immobile. Forming aspergillomas can also be identified by the fungal strands that fall into the cavity lumen, trapping air and initiating the sponge-work appearance. The CT appearance in patients with positive precipitins is characteristic and allows earlier diagnosis than does conventional tomography.