Severe hyperlipidemia was nearly completely corrected in 16 diabetic patients who were treated with regular insulin at breakfast and supper. Serum cholesterol levels fell from 572 ± 52 mg/dl to 247 ± 10 mg/dl and serum triglycerides fell from 6,330 ± 820 mg/dl to 354 ± 40 mg/dl over a 4-month period of treatment. Establishment of comparable degrees of control of the fasting blood glucose and hemoglobin A1clevels by NPH insulin did not correct the hyperlipidemia. Regular insulin timed to act for the disposal of ingested substrates appears to provide physiologic actions important in the treatment of diabetic hyperlipidemia.