OBJECTIVE Insulin autoimmune syndrome (IAS) is an extremely rare cause of hypoglycemia, particularly in non-Asian populations. METHODS In this report, we describe a white male patient with elevated total insulin (>100.0 μIU/mL), C-peptide, and proinsulin levels who was diagnosed with IAS due to anti-insulin antibodies. He also had a small IgG κ M-protein. RESULTS We show that anti-insulin antibodies and/or the monoclonal protein can significantly interfere with insulin and C-peptide immunoassays and propose polyethylene glycol precipitation to quantitate free C-peptide levels as a useful assay in differentiating IAS due to anti-insulin antibodies from insulinoma. CONCLUSIONS In patients presenting with hypoglycemia with excessively high insulin levels, consideration needs to be given to autoimmune hypoglycemia due to anti-insulin antibodies as a cause. Additionally, if total C-peptide levels are increased, free C-peptide needs to be quantitated following polyethylene glycol precipitation.