Aim Few epidemiological data are available describing the sudden death of persons in their prime. This study aims to elucidate when and how sudden death occurs among employees. Methods A total of 196775 employees from 10 workplaces in Central Japan were surveyed for non-traumatic sudden death during 1989–1995. Demographic data and information regarding onset were collected by their workplace healthcare professionals. Results We identified 251 male and 13 female cases of sudden death. The annual incidence was 21·9 (for men) and 5·7 (for women) per 100000 population. Sudden death occurred more frequently in April when the new business year starts (risk ratio [95% confidence interval], 1·62 [0·94–2·79]) than in other months, without seasonality. Sudden death peaked on Sundays (risk ratio, 1·90 [1·20–2·99]) and Saturdays (risk ratio, 1·36 [0·83–2·21]) as compared with weekdays, and was likely to occur in the small hours (risk ratio, 1·71 [0·94–3·10] at 00–0300h and 1·47 [0·79–2·72] at 0300–0600h vs at 0900–1200h. Only 17% of employees died at work, which was significantly less than expected (PConclusion These findings differed from those of elderly people and suggest that sudden death of persons in their prime is related to occupational stress and its relief.