To investigate effects of low body mass index (BMI) and smoking on all-cause mortality among middle-aged and elderly Japanese, we conducted a community-based prospective study. A mail survey was conducted in 1987-1990 in four towns, western Japan. A cohort of 7,301 Japanese men and 8,825 Japanese women was followed up from the date of the mail survey to 1995 in three of the towns and 1998 in the fourth town. We investigated the effect of BMI and smoking on all-cause mortality by using Cox's proportional hazards model. The relationship between BMI and all-cause mortality was a reverse J-shape with minimal mortality in 24 < or = BMI < 26 in men and a U-shape with minimal mortality in 22 < or = BMI < 24 in women, after adjusting for age and smoking. The lowest BMI category (BM < 20) had the highest all-cause mortality in men and also in women. Taking only never-smokers, the highest risk for all-cause mortality was observed in the lowest BMI category for men and for women. This does not seem to be explained by smoking and pre-existing diseases. More attention should be paid to persons with low BMI.