Aspergillus fumigatus produces 2-pentylfuran (2-PF) when cultured on blood agar, nutrient agar and other media. As 2-PF is not known to be produced by mammalian metabolism we hypothesized that it is detectable in breath of patients colonized or infected with A. fumigatus. Breath was tested for 2-PF from normal subjects, those undergoing chemotherapy, and adults at risk of colonization or infection with A. fumigatus because of bronchiectasis, cystic fibrosis, or immune suppression. Breath samples were collected in five L tedlar bags and analyzed by Gas Chromatography/Mass Spectroscopy (GC/MS) in MS-MS mode. 2-PF was not detected in breath 14 healthy controls, in one of 10 neutropenic subjects and 16 of 32 patients with lung disease. The sensitivity and specificity of the 2-PF breath tests when compared with recurrent isolation of aspergillus from sputum or from bronchoalveolar lavage over two months was 77% and 78% respectively. As 2-PF is not normally found in human breath its presence may reflect the active metabolism of A. fumigatus in the airways and form the basis of a new diagnostic breath test for Aspergillus infection.