Forty-two cases of lung cancer complicated with diffuse interstitial pulmonary fibrosis (DIPF) were selected from 13,056 autopsy cases. They were divided into primary (Group I and II) and secondary (Group III) DIPF, and histopathologic and clinicopathologic studies were made. The prevalence of lung cancer with primary DIPF was 17% (8/47) which was significantly (p less than 0.01) higher than that without DIPF (7% or 886/13,009). All eight cases of lung cancer in Group I and II were male, and 7 had cigarette-smoking history. The anatomical sites of lung cancer and DIPF were all peripheral in origin, and in 7 cases located primarily in the lower lobes (left:right = 6:1). Adenocarcinoma was the most common type. The study suggests that male DIPF patients with long clinical history should be followed thoroughly for lung cancer.