The protective effects of TGF-β have been documented in various autoimmune diseases, mostly in organ-specific autoimmunity including type 1 diabetes mellitus (T1DM). However, TGF-β also plays a role as a pro-inflammatory mediator by induction of Th17 cytokine production. IL-23 also plays a key role in differentiation of Th17 cells, which are implicated in pathogenesis of autoimmune conditions including T1DM. The aim of this study was to investigate and compare the difference in the level of TGF-β1 and IL-23 gene expression in unstimulated peripheral blood mononuclear cells (PBMCs) of patients with different forms of diabetes compared with normal healthy controls subjects. Patients with T1DM were grouped as early-onset T1DM (N = 20) with age at diagnosis <18 years and late-onset T1DM (N = 20) with the age at onset >18 years. Patients with T2DM (N = 20) and normal healthy controls (N = 20) were recruited from the same area. TGF-β1 and IL-23 gene expression in fresh unstimulated PBMCs was determined in each group using quantitative real-time PCR. The results confirmed that a significant difference in TGF-β1 and IL-23 gene expression was observed in both forms of juvenile-onset T1DM and adult-onset T1DM compared to the controls and T2DM patients. There was no significant difference for TGF-β gene expression in patients with T2DM and controls. We therefore conclude that our results support the previous data on TGF-β gene down-regulation in T1DM. Also up-regulation of IL-23 has been observed in T1DM whilst it was down-regulated in T2DM. We also found no significant difference between juvenile-onset and adult-onset T1DM indicating same mechanism might be involved in the pathogenesis of both types. More studies on different cytokines in Th17 pathways are required to further confirm our finding.