In some villages in central Turkey pleural changes occur as a result of environmental exposure to mineral fibres. In most cases the fibre is asbestos but in some cases the non-asbestos fibre erionite, a zeolite, is responsible. The incidence of malignant mesothelioma is much higher in "erionite villages" than in "asbestos villages" despite similar frequencies of pleural changes. In this study chest radiographs from 466 people from asbestos villages, 549 from erionite villages, and 382 controls were compared. The frequency of pleural calcification was about the same in the two groups of villages studied, but the minor fissures were visible to a greater degree in erionite cases. In people from erionite villages "atypical" pleural calcification, due to calcification of the visceral rather than the parietal pleura, was more common. These differences may indicate that the fibres have different lengths and diameters.