The clinical features of chronic hypersensitivity pneumonitis (HP) are similar to idiopathic interstitial pneumonias (IIPs) including idiopathic pulmonary fibrosis (IPF). We report 2 cases of chronic summer-type HP with insidious onset. They were misdiagnosed as having IIPs before referral to our hospital. Anti-trichosporon antibodies were positive in these cases. Their disease progressed due to the intermittent or continuous exposure to the antigen. Chronic summer-type HP should be included in the list of differential diagnosis of chronic interstitial lung diseases. Environmental investigation for an accurate diagnosis is important to convince the patient of the necessity to strictly avoid any future exposure to antigen.