The present study evaluated the efficacy of switching from premixed insulin or an insulin analogue to insulin glargine plus oral antidiabetic drugs (OADs) in patients with type 2 diabetes mellitus (T2DM). The feasibility and suitability of the regimen to the patients was examined based on islet function. Patients with T2DM (n=30) treated with stable doses of premixed insulin or an insulin analogue for eight weeks were divided into two groups according to islet function. Group A had a 2 h of C peptide (2hCP)/fasting C peptide (FCP) ratio ≤3, whereas group B had a 2hCP/FCP ratio >3. Eight weeks following the switch to insulin glargine plus OADs, a significant decrease in fasting blood glucose (FBG), 2 h postprandial blood glucose (2hPBG) and glycosylated‑haemoglobin (HbA1c) were observed in the two groups, with effective rates of 75, 42.9 and 39.3%, respectively. A distinct reduction in the insulin dose was particularly evident in group B. There was a marked decrease in FBG in group A, more so than that observed in group B. By contrast, the decrease in HbA1c was more evident in group B following the switch. A larger number of patients in group B had HbA1c≤7.0%, compared with group A. No difference in the incidence of hypoglycaemia and change of body weight were observed. Following the switch to insulin glargine plus OADs, patients with T2DM demonstrated improved blood glucose control and reduced insulin dosage. The results revealed that this switch in regimen is more suitable for patients with T2DM with 2hCP/FCP>3 and that administration of insulin glargine plus OADs is more efficacious for patients with T2DM with increased FBG levels.