Serum KL-6 concentrations in the summer season from June 1 to September 30 and the winter season from November 1 to February 28 were retrospectively analyzed in patients with idiopathic pulmonary fibrosis (IPF, n = 16), non-specific interstitial pneumonia (NSIP, n = 16), collagen vascular disease-associated interstitial pneumonia (CVD-IP, n = 33), house-related HP (House-HP, n = 9), bird-related HP (Bird-HP, n = 9), and combined pulmonary fibrosis and emphysema (CPFE, n = 13). The gene discussed is MUC1; the disease is hypersensitivity pneumonitis.