AIMS: To investigate the association between serum orexin concentrations and insulin resistance/sensitivity in a sample of patients with type 2 diabetes mellitus, and to study the effects of anti-hyperglycemic treatment on orexin concentrations over three months. METHODS: This study was designed as a randomized, open-label, clinical trial. Before allocation, sixty medication-naïve, newly-diagnosed, type 2 diabetes patients underwent a 75 g oral glucose tolerance test (OGTT). Afterwards, using a randomized trial design (IRCT201102275917N1) patients were allocated to either the metformin (1000 mg daily) or pioglitazone (30 mg daily) arm, and were reexamined after three months. Serum insulin, plasma glucose, and orexin concentrations were measured at baseline, during OGTT, and after three months. RESULTS: Orexin concentrations significantly decreased after OGTT (0 vs. 120 min: 0.63 ± 0.07 vs. 0.31 ± 0.03 ng/ml, p < 0.001). Insulin resistance determined by homeostasis model assessment of insulin resistance (HOMA-IR) was significantly and negatively correlated with orexin (r = -0.301, p = 0.024). Furthermore, orexin concentrations were significantly and positively correlated with the insulin sensitivity index derived from OGTT (r = 0.326, p = 0.014). Threemonth treatment with metformin and pioglitazone significantly improved insulin sensitivity and increased orexin concentrations by 26% (p = 0.025) and 14% (p = 0.076), respectively. Between-group analysis showed that changes in orexin concentrations with metformin and pioglitazone were not significantly different (p = 0.742). CONCLUSIONS: There was a negative association between peripheral orexin concentrations and insulin resistance in type 2 diabetes patients. Three-month anti-hyperglycemic treatment with proportionate doses of metformin or pioglitazone increased orexin concentrations via amelioration of insulin resistance and improvement of glycemic control.