Recently, the adverse effects of long working hours on the cardiovascular systems of workers in Japan, including "Karoshi" (death from overwork), have been the focus of social concern. However, conventional methods of health checkups are often unable to detect the early signs of such adverse effects. To evaluate the influence of overtime work on the cardiovascular system, we compared 24-hour blood pressure measurements among several groups of male white-collar workers. As a result, for those with normal blood pressure and those with mild hypertension, the 24-hour average blood pressure of the overtime groups was higher than that of the control groups; for those who periodically did overtime work, the 24-hour average blood pressure and heart rate during the busy period increased. These results indicate that the burden on the cardiovascular system of white-collar workers increases with overtime work.