The Harvard Six Cities Study and results relevant to environmentally related asthma are discussed. The study was initiated in 1974 as a prospective epidemiologic study of the effects of ambient sulfur oxides and particulate matter on the respiratory health of children and adults living in Watertown, Massachusetts; Kingston and Harriman, Tennessee; Steubenville, Ohio; St. Louis, Missouri; Portage, Wisconsin; and Topeka, Kansas. A total of 13,850 school children followed from age 6 to 18, 8,840 adults aged 25 to 74 followed for up to 12 years, and another 6,300 children aged 7 to 11 whose ambient environment has been measured for most of their lives were included in the study. During the 1980/1981 school year, reports of asthma varied from 3.2 to 5.9% in 5,422 white, 10 to 12 year old children. Persistent wheeze was reported in a similar pattern across the cities, with values ranging from 6.6 to 11.6%. In the adults, an increased frequency of persistent wheeze was reported in individuals who reported occupational exposures to dust and fumes. The data suggest that symptoms of persistent wheeze or a history of asthma does identify a subgroup of the population that appears to be at increased risk at levels of ambient exposure at and below the current ambient standards for particulate exposure. The adult data further suggest that cumulative exposure is associated with an increased risk of developing asthma like symptoms.