OBJECTIVE To examine associations between fasting glucose and mortality and to identify the levels associated with lowest mortality by age in diabetes. RESEARCH DESIGN AND METHODS A total of 359,645 Korean adults with known prevalent diabetes participated in health screening during 2001–2004 and were followed up until 2013. RESULTS U-curve associations were found. Fasting glucose levels associated with the lowest mortality were ∼90–130 mg/dL, except for in those aged 18–44 years (∼80–95 mg/dL). Multivariable-adjusted hazard ratios of fasting glucose <65, 65–74, 75–84, 140–169, 170–199, and ≥200 mg/dL were 1.46, 1.12, 1.09, 1.12, 1.31, and 1.78, respectively, compared with 85–99 mg/dL. CONCLUSIONS Optimal fasting glucose range for survival is higher in adults with than without known prevalent diabetes, except, perhaps, younger adults. Tight glucose control may lessen premature death in younger adults with diabetes. Hypoglycemia (<65 mg/dL) was associated with higher mortality than was fasting glucose 170–199 mg/dL, while fasting glucose 65–84 mg/dL had risks comparable with those at levels 140–169 mg/dL in diabetes.