Among 102 patients with idiopathic interstitial pneumonia (IIP), 22 with lung cancer who were treated from 1981 to 1994 were studied retrospectively. As controls, 248 patients with lung cancer only who were treated from 1991 to 1992 were also studied. Lung cancer developed in 6 of 85 patients after IIP was diagnosed (7.1%). The time from the onset of IIP to the onset of lung cancer was 7.7 years. In 17 patients both IIP and lung cancer were present on admission. In general, patients with IIP and lung cancer were heavy smokers. In patients with IIP and lung cancer, %VC was higher than in those with IIP alone, and 20 cancers (90.0%) were in peripheral lung fields. Seventeen patients received anticancer chemotherapy, irradiation, or both. Five patients (29.4%) died of respiratory failure due to progression of pneumonitis after the therapy. The prognosis for patients with IIP and lung cancer was very poor.