Pathological examinations of 233 consecutive autopsy cases with nonasbestos pneumonconiosis revealed evidence of diffuse interstitial fibrosis (DIF) in 64 (27.5%), among whom 45 (19.3%) showed bilateral involvement and 9 (3.9%) extensive disease closely resembling usual interstitial pneumonia. The patients with DIF were significantly older and had longer occupational histories as compared with those without DIF. There was no correlation between the occurrence of DIF and the type of the underlying disease (silicosis or mixed dust pneumoconiosis) except that an extensive DIF was more frequently associated with mixed dust pneumoconiosis. The extensive DIF developed an in situ malignancy much more frequently (33.3%) than the focal disease (2.6%).