Fifty-two bus and truck drivers and a control group of 52 male industrial workers were studied with regard to risk factors for coronary heart disease (CHD) as well as some psycho-social and occupational indicators. The main results were: higher mean values for serum cholesterol, serum triglycerides and blood pressure in the driver group, indicating a markedly higher risk of CHD in the driver group. The two groups were matched for age, socio-economic status, dietary habits, smoking habits and alcohol consumption. In the study the groups were found to be equal as regards CHD-inheritance, physical activity in leisure time, and relative body weight, W/H2. No CO could be traced in the vehicles. The psycho-social and occupational indicators showed considerable differences. The average score for the exposure factor "tempo/time-limit/urgency in job" was 50% higher, and the number of persons with the characteristic "type-A coronary-prone behaviour" was 30% higher in the driver group than in the control group. The difference in blood lipids and blood pressure increased with age. It is suggested that the higher CHD-risk in the driver group is associated with two factors: (1) greater psychic pressure in the working situation; (2) the selection of more type-A persons to driver occupations. A probable, but not commonly accepted theory, is that these psychosocial indicators may influence the traditional risk factors in a harmful way.