A 26-yr-old registered nurse who is employed as a diabetes nurse care coordinator had episodes of symptomatic fasting hypoglycemia associated with hyperinsulinemia and a relatively suppressed plasma C-peptide concentration, arousing the suspicion of self-induced illness. However, using a species-specific radioimmunoassay, the species of insulin during an episode of hypoglycemia was determined to be human in origin. Once the patient's illness was attributed to endogenous insulin hypersecretion, prompt surgical intervention and the diagnosis of nesidioblastosis in an adult was made.