In Japan, Karoshi, which means "death from overwork", has become a focus of social concern. However, no previous study has examined long working hours and job strain simultaneously in relation to incidence of cardiovascular events (CVE) in Japanese workers. We prospectively evaluated the relation between job strain or long working hours and risk of CVE in treated hypertensives during the years 1994-2000. In this study, we followed a sample of 1,615 participants (908 men and 707 women) aged 40-65 yr who were working more than 5 working hours per day. Participants completed questions about work-related conditions and lifestyle variables at baseline, and were free of diagnosed cardiovascular disease, stroke or cancer. Job strain (the combination of job demands and job control) was assessed using a simple questionnaire developed with reference to Karasek's model. The main outcome measure was incidence of CVE. During the mean follow-up period of 5.6 yr, we documented 38 cases of initial CVE. Analyses were conducted using the Cox proportional hazard model. We found a significant association between incidence of CVE and job strain categories, but not long working hours. Multivariate relative risks for CVE in active jobs and high strain jobs overall were 2.89 (95% CI: 1.33-6.28) and 2.45 (95% CI: 0.87-6.93); for men 2.94 (95% CI: 1.29-6.73) and 1.86 (95% CI: 0.51-6.75), and for women 3.97 (95% CI: 0.34-46.88) and 9.05 (95% CI: 1.17-69.86), respectively. In conclusion, active jobs and high strain jobs were associated with increased risk of CVE for treated hypertensive workers.