Background: In Japan, mortality from suicide has peaked around 50 years old among men, with increasing trend after 65 years old, and this peak became more apparent in recent years. Beside this, “psychological autopsy” has revealed depression as one of the most important risk factors for suicide. There is, however, no cohort study which examined the relationship between depressive mood measured by simple method and suicide in middle-aged general population. Methods: In 1989, baseline information was collected by a self-administered questionnaire, and 18, 450 workers were followed up to March 31, 1995. All deaths observed during active service were identified, and when retired, its date was recorded. Among 5, 352 male workers aged between 40 to 54, 11 committed suicide during follow-up period of 5 years. Analysis were carried out by Cox's proportional hazard model, controlling for age. Results: Those who slept 9 hours or more per night demonstrated 12.14-fold risk of suicide compared with those who slept less than 9 hours. Smokers were more likely to commit suicide than non-smokers. Those who answered affirmatively to more than 7 out of 12 questions, which were derived from Zung self-rating depression scale, experienced an increased risk of suicide (RR 9.95; 95%Cl: 1.89-52.44), even after adjusting for other confounding factors. Conclusion: We found an association between depressive mood and subsequent suicide in a middle-aged workers. Detailed observation and follow-up of those with depressive mood should be systematically organized with due attention and caution. J Epidemiol, 2000 ; 10 : 173-178