Interstitial lung diseases (ILDs) represent a large group of different diseases, with a large part comprising idiopathic interstitial pneumonias. Differentiating hypersensitivity pneumonitis (HP), especially its chronic form and other ILDs, is difficult because of similarities in radiological manifestation and clinical course, and the difficulty of identifying causative antigens. We recently experienced a patient with Cladosporium-induced chronic HP that developed in a household environment, but the cause had been misdiagnosed as idiopathic interstitial pneumonia for several years. This case highlighted the need for measures differentiating HP from idiopathic interstitial pneumonia. In this study, we examined fungal exposure in ILDs using an antibody titer in serum to identify possible fungus-related HP. We measured the antibody titer to Cladosporium spp. in 34 patients with various ILDs, 17 patients with bronchial asthma, and 21 control subjects using an immunofluorescence assay. ILDs included HP (5 patients), idiopathic interstitial pneumonias (21 patients), and ILDs with collagen vascular diseases (8 patients). Results showed a significantly higher tendency for high anti-Cladosporium antibody titers in ILD groups (12 patients out of 34 patients), compared to patients with bronchial asthma (0/17) or control subjects (0/21). This increase in antibody titers was observed not only in patients with HP, but also in those with idiopathic interstitial pneumonias and those exhibiting collagen vascular diseases with ILDs. This report highlights the pathogenic role of fungal antigens in various ILDs. In conclusion, fungi commonly observed in our living environment such as Cladosporium could be involved in the development of ILDs.