Because asbestos has been demonstrated to cause lung cancer, the issue regarding safety of other fibers, including man-made vitreous fibers (MMVF), has been raised. We reviewed the available evidence, in particular the epidemiologic data, on MMVF and the risk of respiratory system cancer. Glass fibers (especially glass wool) have been studied most extensively. Taken together, the data indicate that among those occupationally exposed, glass fibers do not appear to increase risk of respiratory system cancer. Of six studies that specifically examined rock and slag wool workers, three reported excesses in respiratory system cancer among such workers. Two of these three studies, however, did not control for cigarette smoking, a powerful predictor of such cancers. There are no published studies, in humans, of refractory ceramic fibers. Future studies evaluating the potential of MMVF to increase risk of respiratory system cancer will not add to existing knowledge if investigators do not address potential confounding by cigarette smoking and other workplace carcinogens.