In the first half of the twentieth century epidemiologic evidence linked elevated incidences of pulmonary fibrosis and cancer with inhalation of chrysotile and crocidolite asbestos, a family of naturally occurring inorganic fibrous materials. As the serpentine and amphibole forms of asbestos were phased out, synthetic vitreous fibers (SVFs; fiber glass, mineral wool, and refractory fiber) became increasingly utilized, and concerns were raised that they too might cause adverse health effects. Extensive toxicological research on SVFs has demonstrated that their pulmonary effects are directly related to fiber dose in the lung over time. This is the result of deposition (thin fibers deposit in the lower lung more efficiently than thick fibers) and lung-persistence ("biopersistence" is directly related to fiber length and inversely related to dissolution and fragmentation rates). In rat inhalation studies, asbestos was determined to be 7- to 10-fold more biopersistent in the lung than SVFs. Other than its effect on biopersistence, fiber composition did not appear to play a direct role in the biological activity of SVFs. Recently, the utilization of man-made organic fibers (MMOFs) (also referred to by some as synthetic organic fibers) has increased rapidly for a variety of applications. In contrast to SVFs, research on the potential pulmonary effects of MMOFs is relatively limited, because traditionally MMOFs were manufactured in diameters too thick to be respirable (inhalable into the lower lung). However, new developments in the MMOF industry have resulted in the production of increasingly fine-diameter fibers for special applications, and certain post-manufacturing processes (e.g., chopping) generate respirable-sized MMOF dust. Until the mid-1990s, there was no consistent evidence of human health affects attributed to occupational exposure to MMOFs. Very recently, however, a unique form of interstitial lung disease has been reported in nylon flock workers in three different plants, and respirable sized nylon shreds (including fibers) were identified in workplace air samples. Whether nylon dust or other occupational exposures are responsible for the development of lung disease in these workers remains to be determined. It is also unknown whether the biological mechanisms that determine the respirability and toxicity of SVFs apply to MMOFs. Thus, it is appropriate and timely to review the current data regarding MMOF workplace exposure and pulmonary health effects, including the database on epidemiological, exposure assessment, and toxicology studies.