RATIONALE: Chronic hypersensitivity pneumonitis (CHP) is a condition that arises following repeated exposure and sensitisation to inhaled antigens. The lung microbiome is increasingly implicated in respiratory disease but to date, no study has investigated the composition of microbial communities in the lower airways in CHP. OBJECTIVE: To characterise and compare the airway microbiome in subjects with CHP, idiopathic pulmonary fibrosis (IPF) and controls. METHODS: We prospectively recruited individuals diagnosed with CHP (n=110), IPF (n=45) and controls (n=28). Subjects underwent bronchoalveolar lavage and bacterial DNA was isolated, quantified by qPCR and the 16S rRNA gene was sequenced to characterise the bacterial communities in the lower airways. MAIN MEASUREMENTS AND RESULTS: Distinct differences in the microbial profiles were evident in the lower airways of subjects with CHP and IPF. At the phylum level, the prevailing microbiota of both IPF and CHP subjects included Firmicutes, Bacteroidetes, Proteobacteria and Actinobacteria. However, in IPF, Firmicutes dominated while the percentage of reads assigned to Proteobacteria in the same group was significantly lower compared to CHP subjects. At the genus level, Staphylococcus was increased in CHP and Actinomyces and Veillonella in IPF. The lower airway bacterial burden in CHP subjects was higher than controls but lower than those with IPF. In contrast to IPF, there was no association between bacterial burden and survival in CHP. CONCLUSIONS: The microbial profile of the lower airways in subjects with CHP is distinct from that of IPF and, notably, bacterial burden in individuals with CHP fails to predict survival.