The medical criteria for initiating insulin therapy, based on clinical profiles of type 2 diabetic patients, have not yet been clearly established. We explored various parameters with 48 type 2 diabetic patients who were taking oral hypoglycemic medication. Among parameters, body mass index (BMI), the fasting plasma glucose level (FPG), and plasma chloride concentration were identified by forward-stepwise discriminant analysis as parameters that can discriminate between patients who were and those who were not undergoing insulin therapy. In combination, these parameters correctly diagnosed 86.4% of the patients who were undergoing insulin therapy, and 84.6% of those who were not undergoing insulin therapy. Further, we observed significant correlations between plasma chloride concentrations and either plasma sodium or organic acid concentrations, suggesting that impaired insulin action may reduce plasma chloride concentrations through changes in plasma sodium and organic acid metabolism. Our results suggest that plasma chloride concentration is a possible new indicator of insulin insufficiency.