Pulmonary fibrosis is a feared sequelae of paracoccidioidomycosis. We sought to determine if mice exposed to Paracoccidioides brasiliensis conidia would develop pulmonary fibrosis. BALB/c mice were infected intranasally with P. brasiliensis conidia and sacrificed at regular intervals. One lung was sectioned for histopathology and sections were stained with haematoxylin and eosin, trichromic and argentic stains; the other lung was homogenized and cultured to determine the viability of the fungus. One week post-challenge, only small peribronchial foci were apparent. After 4 weeks, reticular fibres appeared disorganized and disrupted. Six to 8 weeks later peribronchial infiltrates were larger and appeared surrounded by reticular fibres; thick collagen I fibres were noticed in the infiltrated areas at this time. On weeks 10-12, infiltrates were confluent and reticular fibres were concentrated around the inflammatory foci; collagenization was apparent. Observations up to 16 weeks revealed diffuse involvement of the lung parenchyma with extensive collagenization. Lung cultures were always positive. We suggest that inhalation of P. brasiliensis conidia induces adverse lung responses leading to changes in the proportion of collagen fibres I and III.