Diffuse interstitial lung disease in asbestos-exposed workers is presumed to represent asbestosis. Among 176 asbestos-exposed persons for whom lung tissue was available, we found nine with clinical features consistent with asbestosis, but histologic sections failed to demonstrate asbestos bodies, the usual requirement for pathologic diagnosis of asbestosis (Group I). These nine were compared by analytic electron microscopy with nine persons with idiopathic pulmonary fibrosis (Group II), and with nine persons with all the criteria of asbestosis (Group III). The three groups did not differ significantly with respect to lung burden of chrysotile or tremolite and actinolite, but Group III had a lung burden of amosite and crocidolite that was three orders of magnitude greater than in Groups I and II, with no overlap. We conclude that (1) the American Thoracic Society criterion of "a reliable history of exposure" is sometimes difficult to define; (2) asbestos bodies are seen in tissue sections only when exposure has been reasonably high, and given the proper clinical setting, the presence of diffuse fibrosis and asbestos bodies in tissue sections are sensitive and specific criteria for a diagnosis of asbestosis; and (3) the prevalence here of 5.1% nonasbestos-induced interstitial lung disease among asbestos-exposed persons is artefactually high because of atypical case selection. However, because asbestosis is a disappearing disease, such cases will become more frequent. The identification of these other diseases is important because therapy and prognosis may differ from that of asbestosis.