Man-made vitreous fibres (MMVF) showed carcinogenic potential in experimental animals. Epidemiological data suggested an increased mortality from lung cancer among production workers, but the interpretation is still a matter of controversy. A European study encompassing 13 plants in 7 countries pointed towards a moderate excess of lung cancer among workers employed longer than 1 year in the production of rock/slag wool (SMR = 1.34, 95% CI = 1.08-1.63) and glass wool (SMR = 1.27, 95% CI = 1.07-1.50); the latter increase was not confirmed after applying local rates to calculate expected deaths. The elevated risk among rock/slag wool producers was present even in comparison with local rates, and was associated with increasing time from first exposure, and duration of exposure. Glass wool results exhibited a less definite pattern. Smoking was excluded, although indirectly, as a sufficient alternative explanation of the increased lung cancer risk. In a few plants, exposure to asbestos had occurred in limited periods for some workers, and might have contributed to the findings. Case-control studies are under way to thoroughly investigate the relative and possibly combined role of the different exposures, either occupational or not. Cohort studies in the USA produced results closely consistent with those of the European study.