BACKGROUND So far, the association of lung cancer with chronic hypersensitivity pneumonitis (CHP) has not been studied. OBJECTIVE We examined the prevalence and revealed clinical features of lung cancer in CHP. METHODS We retrospectively reviewed the medical records from 1994 through 2005 and identified 11 patients (15 lesions) with lung cancer among 104 patients with CHP. Their clinical features and histopathological findings were analyzed. RESULTS Ten men and 1 woman with a median age of 68.9 years were included. All patients had a smoking history. The most prevalent histopathological type of lung cancer was squamous cell carcinoma (53%), and all tumors were located in the peripheral region of the lung. Four patients suffered from lung cancer after the diagnosis of CHP and 1 patient had lung cancer before the diagnosis of CHP. The histological pattern of CHP showed a predominantly usual interstitial pneumonia-like lesion. Tumors were located adjacent to honeycombing in 7 (47%) of 15 lesions, bullae in 4 (27%) lesions, and relatively normal lung in 4 lesions. CONCLUSIONS Since the prevalence of lung cancer in CHP seems to be high (10.6%) as seen in idiopathic pulmonary fibrosis, physicians should be aware of the possible complication of lung cancer in CHP.