Idiopathic interstitial pneumonia (IIP) is known to have a tendency to be associated with lung cancer. Clinical characteristics of lung cancer associated with IIP are discussed in this article. Thirteen cases of lung cancer associated with IIP were examined from 1975 to 1988. A total of 590 cases of lung cancer and 38 cases of IIP were observed during that period. The 13 cases of lung cancer were found to be associated with IIP during the follow-up observation of our patients. Of these 13 patients, 12 men and a woman, the average age was 68.0 years old. All these 13 cases were ex- or present smokers. Five cases of lung cancer had no symptoms, but were detected by abnormal shadows on chest X-ray. Eight cases were detected symptoms. The duration from the onset of IIP to the onset of lung cancer was 36.5 +/- 23.5 months. Primary sites of lung cancer were distributed as follows. Nine cancers were in the left lung (64%) and five cancers were in the right lung (36%). Three cancers were in the hilar region (21%) and 11 cancers were in the peripheral lung field (79%). The numbers of lung cancers were equal in upper and lower lobe. The most common histological type was squamous cell carcinoma, followed by adenocarcinoma. Most cases were in stage III or IV. Three cases were operated, 4 were treated with chemotherapy, 4 were treated with chemotherapy plus irradiation, and 2 cases were treated by BRMs. The frequency of weight loss or finger clubbing in these patients was higher than in IIP patients without lung cancer.(ABSTRACT TRUNCATED AT 250 WORDS)