Interstitial pneumonitis associated with interferon alpha therapy for chronic hepatitis C was first described in 1994 by Kazuo et al in Japan. The mechanism of interstitial pneumonitis deveoped by interferon alpha was still unknown but immunologic, allergic or direct lung toxicity were suggested. We experienced a case of interstitial pneumonitis developed during interferon alpha therapy for chronic hepatitis C in a 52-year-old male patient. He was treated with 6 million units of interferon alpha intramuscularly 3 times per week for 4 weeks and noted progressive dyspnea and cough. These symptoms were subsided after 6 weeks' discontinuation of interferon alpha therapy. And so, he was retreated with 3 million units of interferon alpha 3 times per week for 8 weeks and felt dyspnea again. He was admitted to our hospital for further evaluation of progressive dyspnea. Arterial blood gas(ABG) values were PaO2 90.7 mmHg and PaCO2 31.9 mmHg, and antinuclear antibody(ANA) was negative. A chest X-ray film revealed diffuse reticulo-nodular shadows in bilateral lung fields, suggesting a diagnosis of interstitial pneumonitis. A marked increase in lymphocyte count and suppressor T cell were observed in bronchoalveolar lavage(BAL) fluid. Lymphocyte stimulation test with interferon alpha was positive. Interstitial pneumonitis was confirmed by transbronchial lung biopsy. After discontinuation of interferon alpha, we gave oral steroid in the condition that clinical symptoms were being improved gradually.