Concerns engendered by inappropriate extrapolation from rat intracavitary cancer experiments stimulated the initiation of new retrospective and prospective epidemiologic studies of man-made vitreous fibers (MMVF). The results of these new studies have confirmed those of previous investigations that MMVF exposure had not caused an increased risk to develop lung cancer or nonmalignant respiratory disease. In contrast to the high pathogenic potential of MMVF (thin long fibers) when injected into the body cavities of rats, the pulmonary reaction of rodents inhaling such fibers has been that of a nuisance-type dust. The results of new experimental inhalation studies have not yet been published. In vitro studies have demonstrated cytotoxicity of thin long MMVF. Although there is a parallelism between the in vitro cytotoxicity results of MMVF and those of the in vivo intracavitary carcinogenesis studies with the same fibers, it is difficult to attach significance to this parallelism insofar as man is concerned because the rat intracavitary carcinogenesis results have no relevance to man.