The incidence of lung cancer in 45 patients (37 males and 8 females), who were originally diagnosed as having idiopathic interstitial pneumonia (IIP), was studied prospectively by following them 4 to 10 years. As controls, sex-, age-, and smoking history matched patients with chronic obstructive lung disease (COLD) were also surveyed for the same follow up period. In addition, the differences in clinical findings between those with and without lung cancer were examined. Eight out of 45 patients with IIP (18%) developed lung cancer (adenocarcinoma; 4, small cell carcinoma; 3, squamous cell carcinoma; 2, including one double cancer). All lung cancer patients were male smokers. Looking only at males, 8 out of 37 patients with IIP (21.6%) developed lung cancer. This incidence was significantly higher than that of the male COLD group (2.2%, less than 0.01). In clinical findings, IIP patients with lung cancer had the characteristics of the chronic type of IIP (lack of volume loss on chest radiographic milder dyspnea, not on steroid treatment) when compared with those who did not develop lung cancer.