The August, 1991 eruption of Mt. Hudson (Chile) deposited ash across southern Argentina and contributed to the deaths of thousands of grazing sheep. Early ash analysis revealed high levels of fluoride, a potential ash constituent toxic to humans and animals. In order to evaluate fluorosis as the cause of sheep deaths and to examine the possibility that similar ash and airborne toxins could also have an effect on the human population, we conducted an investigation that included health provider interviews, hospital record review, physical examination of sheep, determination of sheep urine fluoride levels, and complete constituent analysis of ash samples collected at proscribed distances from the volcano. Ash deposited farthest from the volcano had highest fluoride levels; all fluoride measurements were normal after rainfall. There were no signs or symptoms of fluorosis observed in sheep or humans. Sheep deaths resulted from physical, rather than chemical properties of the ash.