The devastating 18 May 1980 eruption of Mount St. Helens presented an opportunity to evaluate the acute and potential chronic health effects associated with a major volcanic event. Investigations included assessment of immediate deaths around the volcano, surveillance of hospital visits and admissions in two ash exposure areas in the State of Washington, environmental dust exposures in five affected communities, free silica determinations on several respirable ash samples, and a cross-sectional epidemiological and environmental study of loggers (and unexposed control loggers) likely to have high and prolonged ash exposures. Sixty-three people were killed or missing; an important cause of death was asphyxia from dense ash exposure. Total suspended particulate concentrations in Central and Eastern Washington were quite high in the week following 18 May, but quickly fell to lower levels, except for workers involved in clean-up. Hospital visits and admissions for respiratory disease, especially for asthma, increased following the major eruption, but were not markedly increased in areas affected by a milder second eruption. Loggers were found to have moderate ash exposure which, within six weeks of the major eruption, was associated with an increase in acute respiratory symptoms, but no measurable difference in lung function. Free silica levels of 3 to 7%, as determined from respirable ash samples, suggest some potential for lung damage which is being assessed prospectively.