In this study, histochemistry and immunohistochemistry were used to characterize the phenotype anddistribution of leucocytes in the distal nasal mucosa of 15 dogs with nasal aspergillosis. The mostconsistent histopathological finding was a severe, predominantly lymphoplasmacytic, inflammatoryinfiltration of the lamina propria. Fungal hyphae were not observed to invade the mucosa but were foundat the mucosal surface and within material collected from the nasal cavity. The main immunohistochemicalfindings were (1) a predominance of IgGC plasma cells over IgAC and IgMC plasma cells, (2)significant numbers of macrophages and dendritic cells expressing MHC class II molecules, (3)macrophages and neutrophils expressing L1 antigen and (4) a mixture of CD4C and CD8C Tcells. Thesefindings are consistent with a dominant Th1-regulated cell-mediated immune response. The nature of theinflammatory infiltrate and the lack of invasiveness of the mucosa by the fungus, together with the clinicalcourse of the disease and the apparent immunocompetence of the affected dogs, suggest that canine nasalaspergillosis resembles the chronic erosive non-invasive fungal sinusitis described in human patients.