In May 1995, a 52-year-old man complaining of fever and dyspnea was admitted to a hospital. Based on clinical and radiographic findings, hypersensitivity pneumonitis was suspected. Steroid pulse therapy was unsuccessful, and he was then transferred to our hospital. A chest X-ray film showed bilateral ground-glass shadows and a high-resolution CT scan showed cystic air spaces. The number of CD4-positive lymphocytes in peripheral blood was very low. A test for anti-human immunodeficiency virus antibody was positive and Pneumocystic carinii was found in bronchoalveolar lavage fluid. The acquired immunodeficiency syndrome and Pneumocystis carinii pneumonia were diagnosed. In this patient, the level of serum KL-6, a new marker of interstitial pneumonitis, was very high, and KL-6 was expressed on type II pneumocytes. The level of serum KL-6 may be useful as a marker of the activity of Pneumocystis carinii pneumonia.