OBJECTIVE: Insulin glargine (HOE 901, 21(A)-Gly-30(B)a-L-Arg-30(B)b-L-Arg human insulin) is a novel recombinant analog of human insulin with a shift in the isoelectric point producing a retarded absorption rate and an increased duration of action that closely mimics normal basal insulin secretion. It recently received approval from the Food and Drug Administration. The aim of this study was to evaluate 2 formulations of insulin glargine for safety and efficacy in the treatment of patients with type 1 diabetes. RESEARCH DESIGN AND METHODS: In a 4-week trial, 256 patients with type 1 diabetes received either NPH insulin or insulin glargine containing 30 microg/ml zinc (insulin glargine[30]) or 80 microg/ml zinc (insulin glargine[80]). Insulin glargine was given subcutaneously once daily at bedtime. NPH insulin was given either once daily (at bedtime) or twice daily (before breakfast and at bedtime), according to the patient's prestudy regimen. The initial doses of insulin glargine and NPH were based on the previous NPH total daily dose. RESULTS: At study end point, insulin glargine-pooled groups had significantly lower fasting plasma glucose (FPG) levels than the NPH insulin group, with adjusted mean FPG levels reduced by 2.2 mmol/l (P = 0.0001). Insulin glargine was superior to NPH insulin in reducing FPG levels in patients who had previously received NPH insulin twice daily but not in patients who had previously received NPH once daily. FPG levels were more stable in patients using insulin glargine than in patients using NPH insulin. A subset of patients (n = 71) underwent hourly overnight plasma glucose measurements. Insulin glargine patients exhibited lower FPG levels after 5:00 A.M.; the difference was significant by 8:00 A.M. The adjusted mean FPG for insulin glargine[30] was 7.8 mmol/l; for insulin glargine[80], 7.3 mmol/l; and for NPH, 10.7 mmol/l. Both formulations of insulin glargine were well tolerated, similar to NPH insulin. CONCLUSIONS: Basal insulin glargine administered once daily for 4 weeks as part of a basal-bolus multiple daily insulin regimen was safe and more effective in lowering fasting plasma glucose levels than NPH in patients with type 1 diabetes.