A common treatment regimen for patients with either insulin-dependent diabetes mellitus (IDDM) or non-insulin-dependent diabetes mellitus (NIDDM) is a combination of rapid-acting insulin and intermediate-acting insulin administered twice each day. It is usually recommended that regular human insulin be injected 30 to 45 minutes before a meal. In practice, patients often inject regular human insulin closer to mealtime, causing a higher post-prandial serum glucose level and an increased potential for hypoglycemia in the postabsorptive period. Insulin lispro, a rapid-acting insulin analogue, is best injected just before a meal because of its more rapid absorption and shorter duration of action. In 707 randomized patients, 379 with IDDM and 328 with NIDDM, we studied the effect of twice-daily insulin lispro or regular human insulin in combination with NPH human insulin (isophane insulin) on premeal, 2-hour postprandial, and bedtime glycemic control. Assessments were based on the results of a seven-point blood glucose profile, the insulin dose (by formulation and time of administration), the incidence and frequency of hypoglycemic episodes, and the glycated hemoglobin value. Treatment with insulin lispro resulted in lower postprandial glucose levels and smaller increases in glucose level after the morning and evening meals compared with treatment with regular human insulin. Overall glycemic control, frequency of hypoglycemic events, and total insulin dose were not different between the two groups. Insulin lispro in combination with NPH human insulin in a twice-per-day regimen allows injection closer to mealtime and improves post-prandial glycemic control without increasing the risk of hypoglycemia.