Objectives: This study investigates the volatile (vapour) component of an essential oil derived from the Australian native Leptospermum petersonii as a potential treatment for aspergillosis. Methods: The in vitro antifungal effects of the volatiles were assayed by a variety of methods. In vitro mammalian cell toxicity of the oil and the oil volatiles was also determined prior to animal testing. Efficacy of the volatiles in vivo was assessed using a murine model. Results: L. petersonii oil volatiles were found to be potent inhibitors of fungal growth in vitro, with fungicidal activity displayed following short exposure times ( 1 h). No significant mammalian cell toxicity was found to be associated with the volatiles. In the absence of treatment, Aspergillus fumigatus infection of animals resulted in an increase in inflammatory cell counts and high fungal burden within the lung tissue. Chitin levels in treated animals were significantly reduced compared with control animals. No viable fungi could be recovered from animals that had completed the treatment regimen. Conclusions: The significant reduction in fungal burden in the lungs of infected animals by the volatiles of L. petersonii oil was larger than that reported for conventional antifungal drugs of choice.