Transitioning safely to insulin therapy when oral antidiabetic agents fail to provide adequate glycemic control is a critical aspect of care for the patient with type 2 diabetes mellitus (T2DM). We evaluated the clinical effectiveness of starting patients on a relatively simple regimen of once-daily injections of either biphasic insulin aspart 70/30 (10 min before dinner), NPH insulin (at 10 p.m.), or biphasic human insulin 70/30 (30 min before dinner) in combination with metformin. Enrolled patients had T2DM and inadequate glycemic control (AlC>/=7.5%) on a previous regimen of metformin as monotherapy or in combination with a sulphonylurea. One hundred and forty (140) patients received metformin monotherapy for 4 weeks followed by 12 weeks of combination treatment with metformin and once-daily insulin injections. AlC levels decreased from baseline by 1.1-1.3% for patients in each of the three treatment groups. Overall, FPG values decreased from baseline by 31% (biphasic insulin aspart), 37% (NPH insulin), and 28% (biphasic human insulin). Subjects whose final FPG level was <126 mg/dl experienced the largest decreases in AlC values (-2.3%, -1.9%, -1.8%, respectively). All three treatment regimens were well tolerated. The results indicate that patients with T2DM can safely and effectively begin insulin therapy using once-daily injections of biphasic insulin aspart 70/30, biphasic human insulin 70/30, or NPH insulin in combination with metformin.