OBJECTIVES: This study attempted to clarify the health effects of implementing a 12-hour shift in place of the traditional 8-hour shift in work in a clean room in an electronic parts producing factory. METHODS: Health-check records during a year or longer before and after the shift change were reviewed regarding subjective symptoms, height, body weight, and blood pressure for 189 male workers who had moved to a 12-hour shift and for 16 male workers who remained on an 8-hour shift. RESULTS: The workers changing to a 12-hour shift showed significant increases in subjective symptoms, particularly psychological symptoms defined as related to fatigue and bodyweight gain, 1 kg on average, in the year after the shift change. Increased symptoms and body weight remained high even another year later. The workers remaining in the 8-hour shift did not show significant changes in symptoms or body weight during the observation. No changes in blood pressure were observed in relation to the shift move. CONCLUSIONS: The results suggested that implementing a 12-hour shift caused marked psychological fatigue and unhealthy weight gain among some cleanroom workers and that the adaptation to the new work shift did not seem easy for these workers. Working in a clean room can be considered a job in which special caution is needed regarding increased fatigue when a compressed working schedule is implemented. Attention should also be paid to possible weight gain among persons working 12-hour shifts.