In a survey through pathologists of all fatal malignant mesothelial tumours in North America in 1972, 274 cases were ascertained and case-control studies were carried out. Specimens of lung tissue were obtained for 99 (76 male and 23 female) of the 172 autopsied case-control pairs and were analysed for asbestos fibres by electron microscopy and X-ray energy-dispersive analysis. Equal quantities of chrysotile fibres were found in cases and controls. An amosite fibre content of more than 1×10 6 per g dried lung was found in 26 male cases and 8 controls and of crocidolite in 15 cases and 5 controls. The two men who had ever worked in an asbestos factory, 6 of the 8 who had worked as insulators and 9 of the 14 who had worked in a shipyard had more than 1×10 6 fibres of one or both of these amphiboles. In the 23 female pairs studied, there was a small excess of amphiboles (3 cases and 1 control) and of chrysotile (23 cases and 17 controls) in cases compared with controls.