Previous studies have indicated that urban bus drivers have an increased risk of coronary heart disease. In the present investigation two separate studies were carried out. In the first study the mortality from myocardial infarction (MI), as well as from other causes for all male bus drivers in Sweden (9446), was investigated and compared to those of other employed men over a 15-year period. A 50% increase in mortality from MI was observed among drivers in Stockholm and Göteborg and Bohus counties, the two counties where the two largest cities in Sweden are situated. No increased mortality from lung cancer, any cancer (all sites combined), or from all causes combined was observed for these drivers. For bus drivers in the predominantly rural areas no excess mortality from MI, or any other cause, was observed. In the second study the incidence of MI among male bus drivers, as compared with other employed men, was studied in five Swedish counties by case referent methods. An increased incidence of first events of MI, (relative risk = 1.6, 95% confidence interval (CI) 1.1-1.9) was observed for bus drivers in Stockholm county. According to our results, urban bus drivers thus have an increased risk of developing MI. The findings also suggest that factors in the work environment of urban bus drivers may contribute to this increased risk. Factors of possible importance are job strain, irregular working hours, a sedentary job, automobile exhaust fumes, and noise.