Chronic hypersensitivity pneumonitis (CHP) has a poor prognosis because of the difficulties in its diagnosis and treatment. Recently, we have encountered six cases of CHP (4 patients with the home-related type, a patient with bird fancier's lung, and one with flour-induced CHP), and we examined the clinical characteristics of these cases. Environmental provocation testing has been considered useful for diagnosing HP, but all patients gave negative results in short-term environmental exposure tests performed routinely for the diagnosis of HP. However, 5 patients had a positive response in long-term environmental exposure tests. Two patients died of respiratory failure after ten years' observation despite improvement of the causative environment, and were given steroid therapy. Radiographically and pathologically, the process of progressive and irreversible lesions were recognized in our series of CHP patients. The diagnosis of CHP should be carried out on the basis of the comprehensive findings including the detailed history, clinical course, radiography, and histology.