Pleural thickening, bilateral pleural hyalinised plaques, and restrictive lung function are found among inhabitants of four small villages in northwestern Greece. Transbronchial biopsy samples from patients with disease contained tremolite fibres. Malignant pleural mesothelioma has now been reported in these villages and accounts for approximately 1% of the total mortality from 1981 to 1985. The principal whitewash once used in this area is predominantly asbestiform tremolite. The fibre is identical in every respect to fibres found in the lung tissues of people with Metsovo lung. The membrane activity of this tremolite is greater than that of the commercially used asbestiform amphiboles amosite and crocidolite. This measure of cytotoxicity lends further support to the hypothesis that this fibre is the agent of Metsovo lung and mesothelioma.