We examined 40 autopsy cases of pulmonary asbestosis which were defined by occupational history, lung fibrosis and asbestos bodies (ABs) to clarify histopathological features. Thirty four patients were males and six were females. The mean age was 64.1 +/- 1.6. Gross findings of 39 cases (one case was examined only microscopically) showed that pleural adhesion in 31/39, visceral pleural thickening in 37/39, and pleural plaques in 26/33 except of 6 cases with severe adhesion. Four cases had no or only mild pleural adhesion or pleural thickening, and no pleural plaques. Grossly, lung fibrosis of asbestosis can be divided into two types of honeycombing (HNCB) predominant fibrosis (16 cases), and atelectatic induration predominant fibrosis (23 cases). Histologically, the honeycombing type fibrosis showed peripheral acinar fibrosis like in usual interstitial pneumonia (UIP), whereas the atelectatic induration type exhibited non-peripheral acinar fibrosis with intraluminal organization unlike in UIP. In our study, the lung fibrosis was more intensive in the lower lobes, posterior and subpleural zones, although in eight cases the upper lobes were more intensively involved than the lower lobes. The degree of asbestos body formation on each case was varied. Four cases with typical honeycombing and fibrosis grade 3 were counted only small numbers of asbestos bodies, and lacked one or two above-described pleural changes, and these cases were similar to idiopathic or usual interstitial pneumonia (UIP). As for complications of asbestosis, 13 cases (32.5%) had lung cancer and 14 cases (35%) presented diffuse alveolar damage (DAD) pathologically. It is concluded that asbestosis cases of honeycombing type without pleural changes can not be distinguished even from UIP, if asbestos bodies (ABs) were not found histologically. Therefore, great care needs to be taken in identifying them. We also have examined the quantitatave counts of asbestos bodies and asbestos fibers in ten cases. Acute exacerbation of UIP is a well recognized entity in Japan, similar condition may occur in pulmonary asbestosis.