A 5-year-old girl was given a sulfonylurea hypoglycemic agent, 25 mg of glibenclamide (ten tablets of Euglucon) with two benzodiazepine drugs, 2 mg of estazoram and 0.75 mg of triazolam (one tablet of Eurodin and three tablets of Halcion), by her 37-year-old pharmacist father and then injected with 70 units of insulin (NovoLet 40R). She died several hours after the injection of insulin. Autopsy was carried out 12 h after the death. A glibenclamide level of 103 ng/ml was detected in the serum collected from the heart at autopsy. The serum insulin and C-peptide concentrations were 295 microU/ml and 0.5 ng/ml, respectively. The high level of insulin and the low level of C-peptide indicated that most of the serum insulin was exogenous. The determination of the serum C-peptide concentration was useful to the diagnosis of hypoglycemia caused by exogenous insulin even in the case of co-administration with an endogenous-insulin-releasing agent.