All the male employees of the Italian railroad system at work on April 1, 1963, are classified by age, occupation, level of physical activity, and other relevant social and working characteristics. This group cohort, for a total of 172,459 subjects, is followed for 5 years, and all deaths occurring in that period of time are recorded and analyzed in relation to some of the entry characteristics. The analysis, mainly focused on the question of physical activity at work, shows that the overall corrected death rates are somewhat higher in men with heavy work, as compared with moderate and sedentary workers. The same applies to several other causes of death, individually considered, like other degenerative heart disease (excluding true coronary), chronic bronchitis, and violence. No substantial differences are found between occupations for death rates due to cancer and strokes. The only cause of death which definitely prevails in sedentary and moderate workers, as compared to the heavy ones, is coronary heart disease, defined by myocardial infarction and sudden death of probable coronary origin, differences being statistically highly significant.