A prospective study examined whether working 50 hours or less per week was associated with an increase of mental and circulatory disorders, and if so, whether it was significant for managing workers' health. White-collar workers aged 20 to 60 working at an office of a Japanese company were surveyed from August 1997 to March 2001. The onsets of the diseases were confirmed using medical insurance claims, and the overall incidence rates were 6.6 and 15.7 per 10,000 person months for both the disorders, respectively. Though not significant, those who worked 45 hours or more showed higher relative risks for mental disorders compared with those who did not. No other analysis indicated distinct findings. The small number of onsets of the diseases, the attention to personal health for workers who worked shorter hours, and intensive health care system for circulatory diseases in the workplace may partly account for the findings above. However, through reviewing other reports, it was suggested that the perception of work time could disturb the association between these findings, especially for white-collar workers. Also, it might be useful to observe the onset of mental disorders in work of less than 50 hours with the workers' perception of work time.