Shortly after Mount St. Helens erupted in 1980, a number of laboratories began to investigate the effects of volcanic ash in a variety of experimental systems in attempts to predict effects that might occult in the lungs of humans exposed to volcanic ash. The published results are remarkably consistent, despite the use of non-uniform ash samples and variability in the experimental approaches used. The data indicate that volcanic ash, even in high concentrations, causes little toxicity to lung cells in vitro and in vivo, as compared with effects of free crystalline silica, which is known to be highly fibrogenic. Volcanic ash does not appear to be entirely inert, however, possibly because of low concentrations of free crystalline silica in the ash. The published experimental studies suggest that inhaled volcanic ash is not likely to be harmful to the lungs of healthy humans, but the potential effects of volcanic ash in patients with pre-existing lung diseases are more difficult to ascertain from these studies.