The purpose of this study was to examine the relationship of lung cancer incidence in Contra Costa County to ambient levels of air pollution. It was suspected that the presence of heavy industry in the county, mainly petrochemical plants and oil refineries, could be a contributing factor. Initially, an incidence analysis established that the Industrial portion of the county had an excess of lung cancer as compared to the remaining Non-industrial portion. Air pollution patterns were subsequently determined by five permanent air monitoring stations and ten temporary stations which monitored the levels of 12 air pollutants for a period of one year. By correlating the 1970-79 lung cancer rates for each census tract and tract levels of air pollution constituents, a statistically significant relationship between ambient air SO4 and lung cancer in males, but not in females, was found. However, when adjusted for the percent of the working population categorized as blue collar, the association was eliminated. An interview study of 249 cases and 373 controls was then conducted. Demographic, work history, residential history, dietary, and smoking history questions comprised the bulk of the data collected. Analysis indicated that the major contribution to lung cancer in the county was due to cigarette smoking. No significant association between lung cancer risk and measured constituents of air pollution was found. Of five broad occupational categories (indicating possible hazardous exposures) none had any significant relationship to lung cancer.