The diagnosis of pulmonary aspergillosis is based on serum-analysis, as well as histological and microbiological analysis of bronchial lavage and transbronchial biopsies. When Aspergillus develops within a preformed cavity, however, these tests are likely to be negative. In this situation, classic imaging techniques such as chest X-ray and high resolution-computed tomography (HR-CT) can be of great diagnostic use. We here describe the case of a 62-year-old woman with a history of breast cancer and subsequent ablation of the left breast and radiotherapy. The case demonstrates an example of a pleuropulmonary aspergilloma, in which sero- and micro-biological detection failed. Thorax HR-CT exhibited the cavity, a small persistent pneumothorax, partially filled by an oval density. This density clearly dislocated according to gravity following a positional change of the patient from supine to prone. The density thus revealed mobility which was typical of aspergilloma. Following excision, this diagnosis was confirmed. A density within a cavity may be differentiated by its mobility from differential diagnoses such as lung cancer which would not be expected to exhibit mobility.