The HCO3− concentration in venous serum ([HCO3−]s) is a factor commonly used for detecting the body pH and metabolic conditions. To exactly detect [HCO3−]s, the venous CO2 pressure should be kept as it is in the vein. The [HCO3−]s measurement is technically complicated to apply for huge numbers of almost heathy persons taking only basic medical examinations. The summation of [HCO3−]s and the venous serum Cl− concentration ([Cl−]s) is approximately constant; therefore, we studied if [Cl−]s could be a marker detecting metabolic conditions instead of [HCO3−]s. Venous blood was obtained from persons taking basic medical examinations (the number of persons = 107,630). Older persons showed higher values of [Cl−]s, fasting blood sugar (FBS), and glycated hemoglobin (HbA1c) than younger ones. [Cl−]s showed positive correlation to age and negative correlation to FBS and HBA1c. The negative correlation of [Cl−]s to FBS/HbA1c was obvious in persons with high FBS/HbA1c, leading us to an idea that persons with high FBS/HbA1c show high [HCO3−]s, which might be caused by low activity of carbonic anhydrase in the lung observed in persons with diabetes mellitus under acidotic conditions. Taken together, an easily measured serum electrolyte, [Cl−]s, could be a useful marker estimating metabolic conditions.