Two diabetic patients, who had been treated with human insulin, suffered from fasting hypoglycemia and postprandial hyperglycemia. The insulin-binding capacities of their sera were high, and a large amount of total insulin and prolonged presence of free insulin in the sera were shown. Scatchard analysis of these insulin antibodies revealed that high-affinity insulin antibodies had larger capacity and stronger affinity compared with commonly insulin-treated patients. Treatment with double filtration plasmapheresis and subsequent administration of prednisolone in the second patient reduced such antibodies and resulted in recovery of glycemic control by insulin. Hypoglycemia and hyperglycemia could be incurred when insulin antibodies with strong affinity and high capacity in high-affinity sites arise. This condition can be treated with double filtration plasmapheresis and subsequent administration of prednisolone.