<p>We describe a case of severe hypoglycemia induced by postoperative renal dysfunction associated with prolonged unrecognized cibenzoline intoxication. An 83-year-old woman with type 2 diabetes mellitus (T2DM) developed severe hypoglycemia after orthopedic surgery and was diagnosed with cibenzoline intoxication on the basis of a markedly elevated serum cibenzoline concentration. Cibenzoline discontinuation resolved hypoglycemia and improved bradycardia. Pre-existing bradycardia during cibenzoline therapy and preoperative elevated levels of serum C-peptide were suggestive of unrecognized intoxication. This case provides significant clinical insight by documenting the longitudinal evidence of unrecognized cibenzoline intoxication in a patient with T2DM, which remained latent until the acute manifestation of hypoglycemia. Insulin resistance in diabetes may delay the manifestation of hypoglycemia. While the measurement of cibenzoline concentration is the primary method for monitoring toxicity, the electrocardiogram and C-peptide assessments offer significant supplementary data for diabetic patients. C-peptide measurement is not performed routinely but should be assessed by blood testing when cibenzoline intoxication is suspected from electrocardiographic findings.</p>