In this paper we will briefly review some of the possible techniques for the development of a breath test for aspergillosis and describe progress made toward validating 2-Pentyl furan (2PF) as a marker of Aspergillus infection. Breath testing to diagnose pulmonary aspergillosis is attractive because of the proximity of the lesion to the sample and the simplicity of obtaining diagnostic specimens. Techniques to detect volatile organic compounds (VOCs) in breath discussed include the electronic nose, selective ion flow mass spectrometry (SIFT), ion mobility spectrometry (IMS) and gas chromatography with mass spectrometry (GC/MS). We have used GC/MS to identify Aspergillus-derived VOC's in the head space of cultures. A promising diagnostic marker molecule is 2PF, which was not detectable from cultures of bacterial respiratory pathogens with the possible exception of Streptococcus pneumoniae. 2-Pentylfuran is not known to be produced by mammalian metabolism and was not detectable in the breath of healthy controls, or neutropenic subjects. In contrast, 2PF was found in the breath of patients with lung disease who were colonized or infected with A. fumigatus. Case reports are presented of two severely immune compromised patients with invasive aspergillosis from whom 2PF was detected in multiple breath samples but became undetectable with effective treatment. A well conducted prospective trial is needed to validate the clinical usefulness of this marker for diagnosis and monitoring of invasive aspergillosis. Unanswered questions remaining include how much 2PF, if any, is produced by extensive lung inflammation, and whether food containing high levels of 2PF can cause false positive breath tests either from contamination in the mouth or gastrointestinal absorption and subsequent excretion in the breath.