The aim of this work was to examine whether synergistic associations with mortality exist for BMI and fasting blood glucose (FBG) and to identify FBG-BMI combined subgroups with higher mortality according to sex and age.A total of 15,149,275 Korean adults participated in health examinations during 2003-2006 and were followed up until December 2018. Mortality HRs of 40 FBG-BMI combined groups were assessed by Cox proportional hazards models.During a mean 13.7 years of follow-up, 1,213,401 individuals died. A J-shaped association was seen between FBG and all-cause mortality for all BMI categories. Those with BMI <20 kg/m2 had the highest mortality for any given FBG level, followed by those with BMI 20-22.4 kg/m2. The detrimental effect of elevated FBG was greater among leaner individuals than more corpulent individuals. Moreover, the synergistic adverse effects of hyperglycaemia and leanness was stronger in younger adults than in older adults. Compared with the reference group (overweight with normoglycaemia), age- and sex-adjusted HRs of the leanest with normoglycaemia (BMI <20 kg/m2 and FBG 4.4-5.2 mmol/l), overweight with diabetes (BMI 25-27.4 kg/m2 and FBG ≥10.0 mmol/l) and leanest with diabetes (BMI <20 kg/m2 and FBG ≥10.0 mmol/l) were 1.29, 2.59 and 11.18, respectively, in those aged 18-44 years and 1.56, 1.72 and 2.87, respectively, in those aged 75-99 years. The identification of BMI-FBG subgroups associated with higher mortality was not straightforward, illustrated by the group with FBG 6.1-6.9 mmol/l and BMI 20-22.4 kg/m2 having a similar or higher mortality compared with the group with FBG 7.0-9.9 mmol/l and BMI ≥22.5 kg/m2. In women aged <45 years with FBG <6.9 mmol/l, those with BMI ≥27.5 kg/m2 had the highest mortality, whereas individuals with BMI <20 kg/m2 had the highest mortality for each given FBG level in other age and sex groups.Leanness and hyperglycaemia interact together to increase mortality in a supra-multiplicative manner, especially in younger adults; the interactions of BMI, FBG, sex and age with mortality are complex. Graphical abstract.